John Chrysostom

Or, in that day, on which I shall rise again, ye shall know. For His resurrection it was that established their faith. Then the powerful teaching of the Holy Spirit began. His saying, I am in the Father, expresses His humility; the next, And ye in Me, and I in you, His humanity and God's assistance to Him. Scripture often uses the same words in different senses, as applied to God and to men.

Catena Aurea by Aquinas ↗

Cyril of Alexandria

If it is merely because he loves and is loved that the Son is in God the Father, and if by the same law we are in the Son and he in us, and no different bond of union is discernible—whether we consider that which binds the Son to the Father or us to him and him to us—then in what sense, or on what principle, I ask you, does Jesus say that "in that day" we shall know the mystery of this?…"For in that day you shall know," he says, "that I am in the Father, and you are in me, and I am in you." For I myself live, he says, for I am life …

Commentary on the Gospel of John, Book 9 ↗

Theophylact of Ohrid

"In that day you shall know that I am in My Father," that is, when I rise again, then you shall know that I am not separated from the Father, but have one and the same power. "And you in Me," that is, kept by Me, "and I in you," that is, I am with you, I deliver you from sorrows, I work miracles through you, and in general I glorify you through all things. And in another sense: "I am in you," as the Head in the members, for the apostles are members of Christ (1 Cor. 6:15), "and you in Me," as members in the Head. …

Commentary on John ↗

Augustine of Hippo

"In that day," He says, "ye shall know that I am in my Father, and ye in me, and I in you." In what day, but in that whereof He said, "Ye shall live also"? For then will it be that we can see what we believe. For even now is He in us, and we in Him: this we believe now, but then shall we also know it; although what we know even now by faith, we shall know then by actual vision. …

Tractates on John 75 ↗

Hilary of Poitiers

Now how it is that we are in Him through the sacrament of the flesh and blood bestowed upon us, He Himself testifies, saying, And the world will no longer see Me, but ye shall see Me; because I live ye shall live also; because I am in My Father, and ye in Me, and I in you. …

On The Trinity 8.15 ↗

Thomas Aquinas

Now we see the fruit of his return, which is the knowledge of those things which the apostles did not know. For, as we saw, Peter did not know where Christ was going, and so he asked: "Lord, where are you going?" (13:36); and Thomas did not know this, nor the way he would go: "Lord, we do not know where you are going; how can we know the way?" (14:5). Philip did not know the Father, and so he asked: "Lord, show us the Father, and we shall be satisfied" (14:8). …

Commentary on John ↗