Oecumenius

For there are three that bear witness: the Spirit, the water, and the blood; and these three are one. Therefore, since three bear witness to Jesus' adoption, namely baptism, crucifixion, and resurrection, there is no uncertainty in the Lord's adoption: by which adoption He also granted us, as He who was the first fruits of the entire human mass (Rom. 11:16), to be children of God. And these three are united in one Christ: for this is what it signifies, saying: "These three are one," that is, as a testimony concerning Christ. …

Commentary on 1 John ↗

Augustine of Hippo

I would not have thee mistake that place in the epistle of John the apostle where he saith, "There are three witnesses: the Spirit, and the water, and the blood: and the three are one." Lest haply thou say that the Spirit and the water and the blood are diverse substances, and yet it is said, "the three are one:" for this cause I have admonished thee, that thou mistake not the matter. …

Ten Homilies on 1 John 10 ↗

Leo the Great

"This is He that came by water and blood, Jesus Christ: not by water only, but by water and blood. And it is the Spirit that testifieth, because the Spirit is the truth, because there are three that bear witness, the Spirit, the water and the blood, and the three are one." The Spirit, that is, of sanctification, and the blood of redemption, and the water of baptism: because the three are one, and remain undivided, and none of them is separated from this connection; because the catholic Church lives and progresses by this faith, so …

Letters 28 ↗

Bonaventure

The incarnation cannot be known unless one is aware of the distinction of Persons. For if you do not know the Trinity, the Father could have been incarnated and have suffered as well as the Son: and this is Sabellianism and Patripassianism. Likewise, if you suppose the Trinity and not the incarnation, you have a witnessing in heaven, but you do not receive it on earth. …

Collations on the Hexaemeron, Collation 8 ↗