Irenaeus – Against Heresies, Book 2: Chap. 34 – Irenaeus on the eternality of the soul:
The Lord has taught with very great fulness, that souls not only continue to exist, not by passing from body to body, but that they preserve the same form [in their separate state] as the body had to which they were adapted, and that they remember the deeds which they did in this state of existence, and from which they have now ceased,—in that narrative which is recorded respecting the rich man and that Lazarus who found repose in the bosom of Abraham. In this account He states that Dives knew Lazarus after death, and Abraham in like manner, and that each one of these persons continued in his own proper position, and that [Dives] requested Lazarus to be sent to relieve him—[Lazarus], on whom he did not [formerly] bestow even the crumbs [which fell] from his table. [He tells us] also of the answer given by Abraham, who was acquainted not only with what respected himself, but Dives also, and who enjoined those who did not wish to come into that place of torment to believe Moses and the prophets, and to receive the preaching of Him who was to rise again from the dead. By these things, then, it is plainly declared that souls continue to exist that they do not pass from body to body, that they possess the form of a man, so that they may be recognised, and retain the memory of things in this world; moreover, that the gift of prophecy was possessed by Abraham, and that each class [of souls] receives a habitation such as it has deserved, even before the judgment.
Irenaeus – Against Heresies, Book 3: Chap. 4 – Irenaeus points to universal Catholic Church as the source of unchanged apostolic doctrine:
Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man [depositing his money] in a bank, lodged in her hands most copiously all things pertaining to the truth: so that every man, whosoever will, can draw from her the water of life. For she is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? For how should it be if the apostles themselves had not left us writings? Would it not be necessary, [in that case,] to follow the course of the tradition which they handed down to those to whom they did commit the Churches?
Irenaeus – Against Heresies, Book 3: Chap. 6 – From Ps. 110:1, Irenaeus defends the oneness of God and that both the Father and the Son share this oneness:
Therefore neither would the Lord, nor the Holy Spirit, nor the apostles, have ever named as God, definitely and absolutely, him who was not God, unless he were truly God; nor would they have named any one in his own person Lord, except God the Father ruling over all, and His Son who has received dominion from His Father over all creation, as this passage has it: “The Lord said unto my Lord, Sit Thou at my right hand, until I make Thine enemies Thy footstool.” Here the [Scripture] represents to us the Father addressing the Son; He who gave Him the inheritance of the heathen, and subjected to Him all His enemies. Since, therefore, the Father is truly Lord, and the Son truly Lord, the Holy Spirit has fitly designated them by the title of Lord.
Irenaeus – Against Heresies, Book 3: Chap. 11 – Here, Irenaeus presents an interesting argument for the existence of the 4 gospels:
It is not possible that the Gospels can be either more or fewer in number than they are. For, since there are four zones of the world in which we live, and four principal winds, while the Church is scattered throughout all the world, and the “pillar and ground” of the Church is the Gospel and the spirit of life; it is fitting that she should have four pillars, breathing out immortality on every side, and vivifying men afresh. From which fact, it is evident that the Word, the Artificer of all, He that sitteth upon the cherubim, and contains all things, He who was manifested to men, has given us the Gospel under four aspects, but bound together by one Spirit. As also David says, when entreating His manifestation, “Thou that sittest between the cherubim, shine forth.” For the cherubim, too, were four-faced, and their faces were images of the dispensation of the Son of God. For, [as the Scripture] says, “The first living creature was like a lion,” symbolizing His effectual working, His leadership, and royal power; the second [living creature] was like a calf, signifying [His] sacrificial and sacerdotal order; but “the third had, as it were, the face as of a man,”—an evident description of His advent as a human being; “the fourth was like a flying eagle,” pointing out the gift of the Spirit hovering with His wings over the Church. And therefore the Gospels are in accord with these things, among which Christ Jesus is seated.