True doctrine is found through the succession of the ancient Church

True knowledge is [that which consists in] the doctrine of the apostles, and the ancient constitution of the Church throughout all the world, and the distinctive manifestation of the body of Christ according to the successions of the bishops, by which they have handed down that Church which exists in every place, and has come even unto us, being guarded and preserved without any forging of Scriptures, by a very complete system of doctrine, and neither receiving addition nor [suffering] curtailment [in the truths which she believes]; and [it consists in] reading [the word of God] without falsification, and a lawful and diligent exposition in harmony with the Scriptures, both without danger and without blasphemy; and [above all, it consists in] the pre-eminent gift of love, which is more precious than knowledge, more glorious than prophecy, and which excels all the other gifts [of God].

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXIII

 

No reformation of the Church through schism will ever make up for it’s mischief

He shall also judge those who give rise to schisms, who are destitute of the love of God, and who look to their own special advantage rather than to the unity of the Church; and who for trifling reasons, or any kind of reason which occurs to them, cut in pieces and divide the great and glorious body of Christ, and so far as in them lies, [positively] destroy it,—men who prate of peace while they give rise to war, and do in truth strain out a gnat, but swallow a camel. For no reformation of so great importance can be effected by them, as will compensate for the mischief arising from their schism. He shall also judge all those who are beyond the pale of the truth, that is, who are outside the Church; but he himself shall be judged by no one.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXIII

How can we be saved if Christ wasn’t God?

He will judge also the Ebionites; [for] how can they be saved unless it was God who wrought out their salvation upon earth? Or how shall man pass into God, unless God has [first] passed into man? And how shall he (man) escape from the generation subject to death, if not by means of a new generation, given in a wonderful and unexpected manner (but as a sign of salvation) by God—[I mean] that regeneration which flows from the virgin through faith?

Or how shall they receive adoption from God if they remain in this [kind of] generation, which is naturally possessed by man in this world? And how could He (Christ) have been greater than Solomon, or greater than Jonah, or have been the Lord of David, who was of the same substance as they were?

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXIII

 

One God — two testaments

For all the apostles taught that there were indeed two testaments among the two peoples; but that it was one and the same God who appointed both for the advantage of those men (for whose sakes the testaments were given) who were to believe in God,

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXII

God the Father and His Word are one and the same

For to whomsoever the Lord shall say, “Depart from me, ye cursed, into everlasting fire,” these shall be damned for ever; and to whomsoever He shall say, “Come, ye blessed of my Father, inherit the kingdom prepared for you for eternity,” these do receive the kingdom for ever, and make constant advance in it; since there is one and the same God the Father, and His Word, who has been always present with the human race, by means indeed of various dispensations, and has wrought out many things, and saved from the beginning those who are saved, (for these are they who love God, and follow the Word of God according to the class to which they belong,) and has judged those who are judged, that is, those who forget God, and are blasphemous, and transgressors of His word.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVIII

Christ’s descent to the dead

It was for this reason, too, that the Lord descended into the regions beneath the earth, preaching His advent there also, and [declaring] the remission of sins received by those who believe in Him. Now all those believed in Him who had hope towards Him, that is, those who proclaimed His advent, and submitted to His dispensations, the righteous men, the prophets, and the patriarchs, to whom He remitted sins in the same way as He did to us, which sins we should not lay to their charge, if we would not despise the grace of God. For as these men did not impute unto us (the Gentiles) our transgressions, which we wrought before Christ was manifested among us, so also it is not right that we should lay blame upon those who sinned before Christ’s coming.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVII

The Apostolic faith is through succession and is the key to the Scriptures

Where, therefore, the gifts of the Lord have been placed, there it behoves us to learn the truth, [namely,] from those who possess that succession of the Church which is from the apostles, and among whom exists that which is sound and blameless in conduct, as well as that which is unadulterated and incorrupt in speech. For these also preserve this faith of ours in one God who created all things; and they increase that love [which we have] for the Son of God, who accomplished such marvelous dispensations for our sake: and they expound the Scriptures to us without danger, neither blaspheming God, nor dishonoring the patriarchs, nor despising the prophets.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVII

Gatherings outside of Apostolic succession should be viewed with suspicion

Wherefore it is incumbent to obey the presbyters who are in the Church,—those who, as I have shown, possess the succession from the apostles; those who, together with the succession of the episcopate, have received the certain gift of truth, according to the good pleasure of the Father. But [it is also incumbent] to hold in suspicion others who depart from the primitive succession, and assemble themselves together in any place whatsoever, [looking upon them] either as heretics of perverse minds, or as schismatics puffed up and self-pleasing, or again as hypocrites, acting thus for the sake of lucre and vainglory.

…But those who cleave asunder, and separate the unity of the Church, [shall] receive from God the same punishment as Jeroboam did.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVI

 

God the Son became man to conquer the enemy

As I have pointed out in the preceding book, the apostle did, in the first place, instruct the Gentiles to depart from the superstition of idols, and to worship one God, the Creator of heaven and earth, and the Framer of the whole creation; and that His Son was His Word, by whom He founded all things; and that He, in the last times, was made a man among men; that He reformed the human race, but destroyed and conquered the enemy of man, and gave to His handiwork victory against the adversary.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXI

The three spies a type of the personhood of the Trinity

Thus also did Rahab the harlot, while condemning herself, inasmuch as she was a Gentile, guilty of all sins, nevertheless receive the three spies, who were spying out all the land, and hid them at her home; [which three were] doubtless [a type of] the Father and the Son, together with the Holy Spirit.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XX