Albert Barnes 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. In the time when my life will be fully manifested to you, and you will receive the assurance that I live. This refers to the time after his resurrection, and to the manifestations by which in various ways he would demonstrate that he was alive.

That I am in my Father, etc. That we are most intimately and indissolubly united. (See Barnes on John 10:38).

You in me. That there is a union between us which can never be severed. (See Barnes on John 15:1, also John 15:2-7).