John Calvin Commentary John 14:20

"In that day ye shall know that I am in my Father, and ye in me, and I in you." — John 14:20 (ASV)

At that day Some refer this to the day of Pentecost; but it rather denotes the uninterrupted course, so to speak, of a single day, from the time when Christ exerted the power of his Spirit until the last resurrection. From that time they began to know, but it was a sort of feeble beginning, because the Spirit had not yet worked so powerfully in them.

For the object of these words is to show that we cannot, through indolent speculation, know the sacred and mystical union between us and him, and again, between him and the Father. Instead, the only way of knowing it is when he diffuses his life in us by the secret efficacy of the Spirit. This is the trial of faith, which I recently mentioned.

As to the manner in which this passage was previously abused by the Arians, to prove that Christ is God only by participation and by grace, it is easy to refute their sophistry. For Christ does not speak merely of his eternal essence, but of that Divine power which was manifested in him. As the Father has placed in the Son all fullness of blessings, so, on the other hand, the Son has conveyed himself entirely into us. He is said to be in us, because he clearly shows, by the efficacy of his Spirit, that he is the Author and the cause of our life.