On Baptism by Tertullian
Chapter VI.--The Angel the Forerunner of the Holy Spirit Meaning Contained in the Baptismal Formula.
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. For if |in the mouth of three witnesses every word shall stand:| -- while, through the benediction, we have the same (three) as witnesses of our faith whom we have as sureties of our salvation too -- how much more does the number of the divine names suffice for the assurance of our hope likewise! 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.