The Orthodox knew it first, but that’s beside the point. It’s an Apostolic practice. Only the most historically naive or uninformed would doubt it.
…But I reply, first of all, that infant baptism is not a recent introduction, nor are its origins traceable to the papal church. For I say that it has always been a holy ordinance observed in the Christian church. There is no doctor, however ancient, who does not attest that it has always been observed since the time of the apostles.
I wanted to touch on this point in passing for the sole reason of informing the simple that it is an impudent slander for these fanatics [the Anabaptists] to make others believe that this ancient practice is a recently forged superstition and to feign that it derives from the pope. For the whole ancient church held to infant baptism long before one ever knew about the papacy or had ever heard of the pope.
Besides, I do not ask antiquity to legitimate anything for us unless it is founded on the Word of God. I know that it is not human custom that gives authority to the sacrament, nor does its efficacy depend on how men regulate it. Let us come, therefore, to the true rule of God, of which we have spoken, that is to say, his Word, which alone ought to hold here.
Their view is that one ought to administer baptism only to those who request it, to those who have made a profession of faith and repented. And thus infant baptism is the invention of man, opposed to the word of God.
In order to prove this they cite the passage from Saint Matthew’s last chapter, where Jesus Christ says to his apostles,
”Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit.”
To which they add this sentence from the 16th chapter of Mark:
“Whoever believes and is baptized will be saved.”
That to them seems an invincible foundation.
… We see that our Lord acted the same way toward Abraham with regard to circumcision. For before he conferred this sign on him he received him into his covenant and instructed him in his Word.
But we must now note that when a man is received of God into the fellowship of the faithful, the promise of salvation which is given to him is not for him alone but also for his children. For it is said to him:
“I am thy God, and the God of thy children after thee.”
Therefore the man who has not been received into the covenant of God from his childhood is as a stranger to the church until such time as he is led into faith and repentance by the doctrine of salvation. But at the same time his posterity is also made a part of the family of the church. And for this reason infants of believers are baptized by virtue of this covenant, made with their fathers in their name and to their benefit. Herein, thus, lies the mistake of the poor Anabaptists. For since this doctrine must precede the sacrament, we do not resist it.
JOHN CALVIN, Brief Instruction for Arming All the Good Faithful Against the Errors of the Common Sect of the Anabaptists in John Calvin: Treatises Against the Anabaptists and Against the Libertines, trans., Benjamin Wirt Farley (Grand Rapids: Baker, 1982), 44–47.