Hello Erick,
When you put it as you have, “When did the covenant idea of baptism first arise?”, I would go back before church history to the apostles and the Lord Jesus. I realize what I will say has been part of the standard argumentation – both pro and con – in the discussions of baptism, but the simplicity of it is striking.
When I was converted – a Jew (albeit non-observant) – it was most natural to me to equate the sign of the New Covenant with the sign of the Old, that is, the Abrahamic. Paul says it succinctly, “And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise” (Gal 3:29). It was said by the LORD concerning Abraham and his seed, “This is My covenant, which ye shall keep, between Me and you and thy seed after thee: Every man child among you shall be circumcised” (Gen 17:10).
Zacharias, John the Baptizer’s father, just prior to Christ’s birth, prophesied that God was about “To perform the mercy promised to our fathers, and to remember his holy covenant; the oath which he sware to our father Abraham....” (Luke 1:72, 73). The oath was that in him all nations of the earth be blessed (Gen 12:3), and that He would be a God unto them (Gen 17:7), and they would be His people.
Paul tells us concerning all (Jew
and Gentile) who came to Messiah upon His drawing them, “Know ye therefore that they which are of faith [in Him], the same are the children of Abraham....[and] they which be of faith are blessed with faithful Abraham (Gal 3:7, 9). What Abraham was blessed with was the friendship of God (2 Chr 20:7; Isa 41:8; James 2:23).
If we are indeed then the seed of Abraham, heirs both of the blessing and the stipulations of the covenant made with him, can we neglect putting the sign and seal of the covenant (Rom 4:11) on our children as well as on ourselves, for the command has never been abrogated.
Here is one place where baptism is equated with circumcision, and those who perform it are obeying the command given Abraham:
And ye are complete in him, which is the head of all principality and power:
In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ:
Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead. (Colossians 2:10-12)
Finally it had been thoroughly fulfilled, the
spiritual reality of outward circumcision, which is the circumcision even Jeremiah spoke of,
circumcision of the heart (9:25, 26). Paul talks again of it in Romans 2:28, 29, and the imagery of it is oft repeated in his epistles, putting off the old man, putting on the new (Eph 4:22-24), putting on Christ (Gal 3:27; Rom 13:12-14), and then Romans 6 which goes into depth, showing that in union with Christ this New Testament “circumcision of the heart” is effected by our being put to death in the flesh – “buried with [Christ] by baptism into death” (6:4a) – and quickened into “new creatures” by the Holy Spirit – “that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life” (6:4b; cf. 8:10, 11). Please note,
this pertains to believers only, not infants, as Paul is talking of all these things being reckoned true
by faith. Peter explicitly says God purifies our hearts “by faith” (Acts 15:9). We baptize our children because, as Abraham’s seed, we believe, and obey the covenant commands.
It will not do to say that the Old Covenant was discontinued and the spiritual truths circumcision signified have no continuance in the New Covenant, as this betrays a confusion of the covenants. Indeed the Mosaic Covenant was superceded by the New, but the Mosaic is not to be confused with the Abrahamic. Paul sharply distinguishes between the two in Galatians 3, asserting that the continuing validity of God’s covenant with Abraham was not annulled by the introduction of Mosaic Law, but remained intact and operative, and that even after the Mosaic Covenant was fulfilled and superceded (verse 17).
In sum: we who believe into Christ, thus being the heirs of Abraham, take baptism as the New Covenant and
spiritual sign and seal, signifying that circumcision of the heart effected by our union with Christ in His death and resurrection.