The Institution of Christian Religion

Now this distinction doth so not stand against the single unity of God, §19that thereby we may prove that the Son is one God with the father, because he hath one Spirit with him, and that the Holy Spirit is not a thing divers from the Father & the Son. For in each Hypostasis is understanded the whole substance, with this that every one hath his own property. The Father is whole in the Son,¹ & the Son is whole in the Father, as himself affirmeth. I am in the Father, and the Father is in me. And the Ecclesiastical writers do not grant the one to be severed from the other by any difference of essence.² By these names that betoken distinction (saith Augustin) that is meant whereby they have relation one to an other, and not the very substance whereby they are all one. By which meaning are the sayings of the old writers to be made agree, which otherwise would seem not a little to disagree. For sometime they say that the Father is the beginning of the Son, and sometime that the Son hath both godhead and essence of himself,³ and is all one beginning with the Father. The cause of this diversity Augustine doth in an other place well and plainly declare, when he sayeth: CHRIST having respect to him self, is called God, and to his Father is called the Sonne. And again▪ ⟨p. 38⟩the Father as to himself is called God,⁴ as to his Son is called the Father, where having respect to the Son he is called the Father, he is not the Son: & where as to the Father he is called the Son, he is not the Father: and where he is called as to him self the Father, and as to himself the Son: it is all one God. Therefore when we simply speak of the Son: without having respect to the Father, we do well and properly say, that he is of him self: and therefore we call him but one beginning: but when we make mention of the relation between him and his Father, then we rightly make the Father the beginning of the Sonne. All the whole fifth book of Augustine concerning the Trinity doth nothing but set forth this matter. And much safer it is to rest in that relation that he speaketh of, than into subtlety piercing unto the high mystery to wander abroad by many vain speculations.