At that day ye shall know that I am {i} in my Father, and ye in me, and I in you.

(i) The Son is in the Father in such a way that he is of one selfsame substance with the Father, but he is in his disciples in a different way, as an aider and helper of them.

People's New Testament

14:20 At that day ye shall know. That day began on Pentecost. There was no more doubting after the Holy Spirit was sent. That day still comes to every soul that wholly surrenders to the will of Christ (Joh 7:17).

Wesley's Notes

14:20 At that day - When ye see me after my resurrection; but more eminently at the day of pentecost.

Jamieson-Fausset-Brown Bible Commentary

20. At that day-of the Spirit's coming.

ye shall know that I am in my Father, ye in me, I in you-(See on [1852]Joh 17:22,23).

Matthew Henry's Concise Commentary

14:18-24 Christ promises that he would continue his care of his disciples. I will not leave you orphans, or fatherless, for though I leave you, yet I leave you this comfort, I will come to you. I will come speedily to you at my resurrection. I will come daily to you in my Spirit; in the tokens of his love, and visits of his grace. I will come certainly at the end of time. Those only that see Christ with an eye of faith, shall see him for ever: the world sees him no more till his second coming; but his disciples have communion with him in his absence. These mysteries will be fully known in heaven. It is a further act of grace, that they should know it, and have the comfort of it. Having Christ's commands, we must keep them. And having them in our heads, we must keep them in our hearts and lives. The surest evidence of our love to Christ is, obedience to the laws of Christ. There are spiritual tokens of Christ and his love given to all believers. Where sincere love to Christ is in the heart, there will be obedience. Love will be a commanding, constraining principle; and where love is, duty follows from a principle of gratitude. God will not only love obedient believers, but he will take pleasure in loving them, will rest in love to them. He will be with them as his home. These privileges are confined to those whose faith worketh by love, and whose love to Jesus leads them to keep his commandments. Such are partakers of the Holy Spirit's new-creating grace.