"Because I live, and you will live." Therefore our communion will be entirely possible and necessary, as between the living.
"I live" does not mean only that after dying I will come to life or rise again, but that I live continuously, not subject to death. Therefore His death does not interrupt His life. Even in death He lives as the source of life, and death serves for Him only as a transition to the heavenly glory of His life, instead of His humble and self-abased life on earth.
"And you will live," that is, in union with Me as the source of life, you will live in the spirit. For the source of life is communion with God, and one who has such communion does not die spiritually.
By the power of this spiritual communion with Him, the source of life, they, being spiritually alive, will see Him or will behold Him in spirit. The unbelieving world, which has no communion with Him and therefore is not alive, will not see Him.
This spiritual vision of the Lord by them, or spiritual union with Him, will begin for them in its fullness with the sending down of the Holy Spirit on the day of Pentecost. On that day, or from that day, the Lord says, the mystery of My union with the Father and with you will be revealed to you.
You will know, so to speak, by experience, through the special enlightenment given you by the Holy Spirit, and not only by faith in My word. You will know that I am in My Father, cf. Jn 10:38, or that I and the Father are one, Jn 10:30, that is, one in essence and therefore in complete moral unity with Him.
You will know that you are in Me, that is, that you have the closest moral union with Me, and that I am in you by the power of this moral union, cf. Jn 15:1-7.
"When you hear the words, I am in the Father, and you in Me, and I in you, do not understand them in one and the same meaning. For the Son is in the Father as consubstantial, but He is in the apostles as helper and fellow-worker. The apostles are in Him as receiving from Him help, cooperation, and zeal" (Theophylact, cf. Chrysostom).