I was reading St. Cyprian of Carthage's first treatise online (http://www.newadvent.org/fathers/050701.htm) and I found these quotations interesting:
"how can a man say that he believes in Christ, who does not do what Christ commanded him to do? Or whence shall he attain to the reward of faith, who will not keep the faith of the commandment? He must of necessity waver and wander, and, caught away by a spirit of error, like dust which is shaken by the wind, be blown about; and he will make no advance in his walk towards salvation, because he does not keep the truth of the way of salvation." - Ch. 2 I find it interesting here how he describes man walking towards salvation, and calls the one church 'the way of salvation'.
"[The Devil] seeing his idols forsaken, and his lanes and his temples deserted by the numerous concourse of believers— to devise a new fraud, and under the very title of the Christian name to deceive the incautious? He has invented heresies and schisms, whereby he might subvert the faith, might corrupt the truth, might divide the unity... He snatches men from the Church itself; and while they seem to themselves to have already approached to the light, and to have escaped the night of the world, he pours over them again, in their unconsciousness, new darkness; so that, although they do not stand firm with the Gospel of Christ, and with the observation and law of Christ, they still call themselves Christians, and, walking in darkness, they think that they have the light, while the adversary is flattering and deceiving, who, according to the apostle's word, transforms himself into an angel of light, and equips his ministers as if they were the ministers of righteousness" - Ch. 3 it's interesting how he says "law of Christ" it goes against the Lutheran Law/Gospel divide, and shows the patristic support for the Catholic Old Law/New Law emphasis.
"If any one consider and examine these things, there is no need for lengthened discussion and arguments. There is easy proof for faith in a short summary of the truth. The Lord speaks to Peter, saying, I say unto you, that you are Peter; and upon this rock I will build my Church, and the gates of hell shall not prevail against it. And I will give unto you the keys of the kingdom of heaven; and whatsoever you shall bind on earth shall be bound also in heaven, and whatsoever you shall loose on earth shall be loosed in heaven. And again to the same He says, after His resurrection, Feed my sheep. And although to all the apostles, after His resurrection, He gives an equal power, and says, As the Father has sent me, even so send I you: Receive the Holy Ghost: Whose soever sins you remit, they shall be remitted unto him; and whose soever sins you retain, they shall be retained; John 20:21 yet, that He might set forth unity, He arranged by His authority the origin of that unity, as beginning from one. Assuredly the rest of the apostles were also the same as was Peter, endowed with a like partnership both of honour and power; but the beginning proceeds from unity. Which one Church, also, the Holy Spirit in the Song of Songs designated in the person of our Lord, and says, My dove, my spotless one, is but one. She is the only one of her mother, elect of her that bare her. Song of Songs 6:9 Does he who does not hold this unity of the Church think that he holds the faith? Does he who strives against and resists the Church trust that he is in the Church, when moreover the blessed Apostle Paul teaches the same thing, and sets forth the sacrament of unity, saying, There is one body and one spirit, one hope of your calling, one Lord, one faith, one baptism, one God? Ephesians 4:4" - Ch. 4 Interesting here, definately not a modern Catholic exegesis, St. Cyprian argues for the equality of the bishops, but it's funny that in a question of the truth he says it is not necessary for lengthy debates, but merely to look to the Church where truth is taught. Again ecclesiology is the basis of everything.
"...also the Church, shone over with the light of the Lord, sheds forth her rays over the whole world, yet it is one light which is everywhere diffused, nor is the unity of the body separated. Her fruitful abundance spreads her branches over the whole world. She broadly expands her rivers, liberally flowing, yet her head is one, her source one; and she is one mother, plentiful in the results of fruitfulness: from her womb we are born, by her milk we are nourished, by her spirit we are animated." - Ch. 5 Here St. Cyprian teaches that this one church feeds and nourishes the faithful in a very Catholic view of mediated salvation.
"Whoever is separated from the Church and is joined to an adulteress, is separated from the promises of the Church; nor can he who forsakes the Church of Christ attain to the rewards of Christ. He is a stranger; he is profane; he is an enemy. He can no longer have God for his Father, who has not the Church for his mother. If any one could escape who was outside the ark of Noah, then he also may escape who shall be outside of the Church. The Lord warns, saying, He who is not with me is against me, and he who gathers not with me scatters. Matthew 12:30 He who breaks the peace and the concord of Christ, does so in opposition to Christ; he who gathers elsewhere than in the Church, scatters the Church of Christ. The Lord says, I and the Father are one; John 10:30 and again it is written of the Father, and of the Son, and of the Holy Spirit, And these three are one. 1 John 5:7 And does any one believe that this unity which thus comes from the divine strength and coheres in celestial sacraments, can be divided in the Church, and can be separated by the parting asunder of opposing wills? He who does not hold this unity does not hold God's law, does not hold the faith of the Father and the Son, does not hold life and salvation." - Ch. 6 Again anyone breaking the unity of the Church CANNOT be serving Christ according to Cyprian. I like the phrase "celestial sacraments".
"even here, before the day of judgment, the souls of the righteous and of the unrighteous are already divided, and the chaff is separated from the wheat. These are they who of their own accord, without any divine arrangement, set themselves to preside among the daring strangers assembled, who appoint themselves prelates without any law of ordination, who assume to themselves the name of bishop, although no one gives them the episcopate" - Ch. 10 Again the episcopate is primary any church without a share in the one episcopate is de facto heretical
"Although there can be no other baptism but one, they (Heretics) think that they can baptize" -Ch. 11 A noteworthy point, here he contradicts soon to be magesterial teaching, Pope Cornelius and eventually Augustine in his view that the sacrament of baptism is only valid within the Catholic Church.
"Nor let any deceive themselves by a futile interpretation, in respect of the Lord having said, Wheresoever two or three are gathered together in my name, there am I in the midst of them. Matthew 18:20 Corrupters and false interpreters of the Gospel quote the last words, and lay aside the former ones, remembering part, and craftily suppressing part: as they themselves are separated from the Church, so they cut off the substance of one section. For the Lord, when He would urge unanimity and peace upon His disciples, said, I say unto you, That if two of you shall agree on earth touching anything that you shall ask, it shall be given you by my Father which is in heaven. For wheresoever two or three are gathered together in my name, I am with them; showing that most is given, not to the multitude, but to the unanimity of those that pray. If, He says, two of you shall agree on earth: He placed agreement first; He has made the concord of peace a prerequisite; He taught that we should agree firmly and faithfully....How can two or three be assembled together in Christ's name, who, it is evident, are separated from Christ and from His Gospel? For we have not withdrawn from them, but they from us; and since heresies and schisms have risen subsequently, from their establishment for themselves of diverse places of worship, they have forsaken the Head and Source of the truth." -Ch. 12 An interesting early church exegesis of this passage used as a prooftext by non-Catholics, again he asserts that the heretics cannot even agree among themselves (sound familiar?) and that this disunity is a mark of heresy.
"What sacrifices do those who are rivals of the priests think that they celebrate? Do they deem that they have Christ with them when they are collected together, who are gathered together outside the Church of Christ?" - Ch. 13 interesting that already in the mid 3rd century there was a belief in the Christian priesthood and the sacrifice of the mass/eucharistic sacrifice
"Even if such men were slain in confession of the Name, that stain is not even washed away by blood: the inexpiable and grave fault of discord is not even purged by suffering. He cannot be a martyr who is not in the Church; he cannot attain unto the kingdom who forsakes that which shall reign there. Christ gave us peace; He bade us be in agreement, and of one mind. He charged the bonds of love and charity to be kept uncorrupted and inviolate" - Ch. 14 A very harsh passage condemning heretical martyrs.
"There is need of righteousness, that one may deserve well of God the Judge; we must obey His precepts and warnings, that our merits may receive their reward." - Ch. 15 An interesting note on Merit the apparently Scholastic (or Tridentine) Dogma that somehow slipped into this ante-nicene writing.
"Yet let not the excessive and headlong faithlessness of many move or disturb us, but rather strengthen our faith in the truthfulness which has foretold the matter...Does he think that he has Christ, who acts in opposition to Christ's priests, who separates himself from the company of His clergy and people? He bears arms against the Church, he contends against God's appointment. An enemy of the altar, a rebel against Christ's sacrifice, for the faith faithless, for religion profane, a disobedient servant, an impious son, a hostile brother, despising the bishops, and forsaking God's priests, he dares to set up another altar, to make another prayer with unauthorized words, to profane the truth of the Lord's offering by false sacrifices, and not to know that he who strives against the appointment of God, is punished on account of the daring of his temerity by divine visitation." -Ch. 17 Very interesting passage where he again mentions the priesthood, the sacrificial nature of the eucharist (twice), and the fact that the liturgy must be authorized.
"they imitate and follow, who, despising God's tradition, seek after strange doctrines, and bring in teachings of human appointment, whom the Lord rebukes and reproves in His Gospel, saying, You reject the commandment of God, that you may keep your own tradition. Mark 7:9" - Ch. 19 An interesting thought on Tradition, that every group heretical and orthodox has Traditions, but some are "God's tradition" and others are human. So much for this verse as an anti-tradition prooftext for Cyprian.
"Solomon, who, although so long as he walked in God's ways, retained that grace which he had received from the Lord, yet after he forsook the Lord's way he lost also then Lord's grace. And therefore it is written, Hold fast that which you have, lest another take your crown. Revelation 3:11 But assuredly the Lord would not threaten that the crown of righteousness might be taken away, were it not that, when righteousness departs, the crown must also depart." - Ch. 20 Here he teaches the loss of salvation, and by connection intrinsic righteousness.
"He that endures to the end, the same shall be saved, Matthew 10:22 whatever has been before the end is a step by which we ascend to the summit of salvation, not a terminus wherein the full result of the ascent is already gained" -Ch. 21 again progressive justification and intrinsic rightousness
"God is one, and Christ is one, and His Church is one, and the faith is one, and the people is joined into a substantial unity of body by the cement of concord. Unity cannot be severed; nor can one body be separated by a division of its structure, nor torn into pieces" - Ch. 23 So much for branch theory, Cyprian will have none of it.
The most interesting one:
"But in us unanimity is diminished in proportion as liberality of working is decayed. Then they used to give for sale houses and estates; and that they might lay up for themselves treasures in heaven, presented to the apostles the price of them, to be distributed for the use of the poor. But now we do not even give the tenths from our patrimony; and while our Lord bids us sell, we rather buy and increase our store. Thus has the vigour of faith dwindled away among us; thus has the strength of believers grown weak. And therefore the Lord, looking to our days, says in His Gospel, When the Son of man comes, think you that He shall find faith on the earth? Luke 18:8 We see that what He foretold has come to pass. There is no faith in the fear of God, in the law of righteousness, in love, in labour; none considers the fear of futurity, and none takes to heart the day of the Lord, and the wrath of God, and the punishments to come upon unbelievers, and the eternal torments decreed for the faithless. That which our conscience would fear if it believed, it fears not because it does not at all believe. But if it believed, it would also take heed; and if it took heed, it would escape." Ch. 26 St. Cyprian's quasi-communist / patristic view on private property (possibly contradicted by later Catholic teaching on the inviolability of private property rights). But very interesting in our modern world of Adam Smith to see this church father measuring faithfulness to the gospel with care for the poor.
All in all it isn't a completely Magesterial Catholic treatise, or Orthodox one, but it certainly isn't a Protestant one. I recommend it.