Begin.
Since I don't know if any particular issue or doctrine is of greater interest to you, I will begin with the Eucharist. It is the defining doctrine of the Catholic faith, it is Jesus Christ.
I'm not sure if you've been around for threads in which we've discussed the Biblical support for the Eucharist, but I'll provide that, too, if you need it. If this doctrine is of no interest to you then give me a topic you'd like to cover and I'll do my best.
Ignatius of Antioch
"I have no taste for corruptible food nor for the pleasures of this life. I desire the bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire his blood, which is love incorruptible" (Letter to the Romans 7:3 [A.D. 110]).
"Take note of those who hold heterodox opinions on the grace of Jesus Christ which has come to us, and see how contrary their opinions are to the mind of God. . . . They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ, flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes" (Letter to the Smyrnaeans 6:2–7:1 [A.D. 110]).
Justin Martyr
"We call this food Eucharist, and no one else is permitted to partake of it, except one who believes our teaching to be true and who has been washed in the washing which is for the remission of sins and for regeneration [i.e., has received baptism] and is thereby living as Christ enjoined. For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by him, and by the change of which our blood and flesh is nurtured, is both the flesh and the blood of that incarnated Jesus" (First Apology 66 [A.D. 151]).
Cyprian of Carthage
"He [Paul] threatens, moreover, the stubborn and forward, and denounces them, saying, ‘Whosoever eats the bread or drinks the cup of the Lord unworthily, is guilty of the body and blood of the Lord’ [1 Cor. 11:27]. All these warnings being scorned and contemned—[lapsed Christians will often take Communion] before their sin is expiated, before confession has been made of their crime, before their conscience has been purged by sacrifice and by the hand of the priest, before the offense of an angry and threatening Lord has been appeased, [and so] violence is done to his body and blood; and they sin now against their Lord more with their hand and mouth than when they denied their Lord" (The Lapsed 15–16 [A.D. 251]).
Cyril of Jerusalem
"The bread and the wine of the Eucharist before the holy invocation of the adorable Trinity were simple bread and wine, but the invocation having been made, the bread becomes the body of Christ and the wine the blood of Christ" (Catechetical Lectures 19:7 [A.D. 350]).
I believe I have said ad nauseum that my primary goal on this religion forum is to defend the Catholic Church against those who would say that our doctrines were "made up" at such and such a date, by so and so. I believe sincere and reasonable people can disagree about what Scriptures say, which brings me right on back to the authority I believe the Lord established.
Bringing the early church fathers into the discussion is meant to support that today's Catholic doctrines were taught in the 1st and 2nd centuries, and onward. Naturally, there were disagreements and heresies, and when there were, councils were held. However, I guess that is history for another day.
I asked Starman and IT earlier to provide me with writings to establish proof that there was an invisible church such as they think existed. Heros and martyrs for Bible only faith who stood up to the supposed heresies of the RCC. Letters to the flocks, etc. The early Christians had no Amazon.com to pick up their own Bible so they must've relied on holy teachers; there must be a paper trail somewhere.