Not that in the waters we obtain the Holy Spirit; but in the water, under (the witness of) the angel, we are cleansed, and prepared for the Holy Spirit. In this case also a type has preceded; for thus was John beforehand the Lord’s forerunner, “preparing His ways.” Thus, too, does the angel, the witness of baptism, “make the paths straight” for the Holy Spirit, who is about to come upon us, by the washing away of sins, which faith, sealed in (the name of) the Father, and the Son, and the Holy Spirit, obtains.
Moreover, after the pledging both of the attestation of faith and the promise of salvation under “three witnesses,” there is added, of necessity, mention of the Church; inasmuch as, wherever there are three, (that is, the Father, the Son, and the Holy Spirit, ) there is the Church, which is a body of three.
Tertullian
On Baptism
Chapter VI