15 Now how it is that we are in Him through the sacrament of the flesh and blood bestowed upon us, He Himself testifies, saying, And the world will no longer see Me, but you shall see Me; because I live you shall live also; because I am in My Father, and you in Me, and I in you. If He wished to indicate a mere unity of will, why did He set forth a kind of gradation and sequence in the completion of the unity, unless it were that, since He was in the Father through the nature of Deity, and we on the contrary in Him through His birth in the body, He would have us believe that He is in us through the mystery of the sacraments? And thus there might be taught a perfect unity through a Mediator, while, we abiding in Him, He abode in the Father, and as abiding in the Father abode also in us; and so we might arrive at unity with the Father, since in Him Who dwells naturally in the Father by birth, we also dwell naturally, while He Himself abides naturally in us also.
Source: On the Trinity (New Advent)