It is indeed true that, just like Pre-Malachi Israel, also the Post-Apostolic Christian Church only progressively succeeded in trying to plumb the depths of the Triune God (cf. First Corinthians 2:9-11). But for that matter, what candid Trinitarian — or even Unitarian — could claim that he even today knows God exhaustively, or ever shall do? Yet the Pre-Nicene Church, from Easter Sunday onward baptizing her converts into the name of the Father and the Son and the Holy Spirit, was always solidly trinitarian.
Thus, in The Teaching of the Twelve Apostles, around A.D. 95, one reads:1 "You shall love the Lord Who made you!…. You shall not practise witchcraft!…. Do not be an observer of omens, since it leads the way to idolatry!…. Baptize into the name of the Father and of the Son and of the Holy Spirit in living water!…. Pour out water thrice upon the head, into the name of Father and Son and Holy Spirit!"