Cyprian

The Lord says, "I and the Father are one;" and again it is written of the Father, and of the Son, and of the Holy Spirit, "And these three are one." And does any one believe that this unity which thus comes from the divine strength and coheres in celestial sacraments, can be divided in the Church, and can be separated by the parting asunder of opposing wills? He who does not hold this unity does not hold God's law, does not hold the faith of the Father and the Son, does not hold life and salvation.

Treatise 1. On the Unity of the Church ↗

Bede

And (he says) the three are one. For these remain indivisible, and none of them is separated from its connection, because divinity is not to be believed without true humanity, nor humanity without true divinity. But in us also these are one, not by nature of the same substance, but by the operation of the same mystery. …

Commentary on the Catholic Epistles ↗

Bonaventure

The words also testify to this same thing, among which, although many there be, that one statement is express which the friend of the Bridegroom wrote in First John five: There are three who give testimony in heaven, the Father, the Word, and the Holy Spirit, and these three are one. Now this testimony is so efficacious and express that it not only makes the aforesaid truth credible, that is, fitting to be believed, but also makes it obligatory, inasmuch as it binds and constrains us to believing it.

Disputed Questions on the Mystery of the Trinity, Question 1 ↗