Some understand our Saviour here speaking of the day of his resurrection; others interpret it of the days of Pentecost, when there should be such an effusion of the blessed Spirit: but the following words discover, that it is best interpreted of the day of judgment, and the general resurrection: for they speak of two great mysteries, which the disciples should understand in that day which is here spoken of; to wit, the personal union of Christ with his Father, and the mystical union of believers with Christ: as to both which, though the apostles and believers knew much more after Christ's resurrection, and the pouring forth of the Spirit in the days of Pentecost, than they knew before those times; yet it is a very imperfect knowledge they ever had, or yet have, of those mysterious unions; but in the resurrection we shall understand these things clearly.
Related Commentaries of John 14:20
John 14:20
20 At that day ye shall know that I am in my Father, and ye in me, and I in you.