Unfortunately this is a passage of Scripture misapplied in the contemporary Ecumenical Movement as a call for divided Christians to come together, when in fact it is clearly a prayer of our Lord that the disciples of Jesus may be united in their existing unity through their glorification by the Holy Spirit. As Fr. John Romanides writes: "That John 17 can be applied to Churches which have not the slightest understanding of glorification (theosis) and how to arrive at this cure in this life is very interesting, to say the least... In John 17 Christ prays for the cure of the glorification of His disciples and their disciples, not for divided Churches, indeed not for traditions which have not the slightest idea of what the cure of glorification is." St. Cyril of Alexandria confirms this when he says in the words of Christ: "This favor and glory then, He says, given unto Me, O Father, by Thee, that is, the glory of being One with Thee, I have given unto them, that they may be one, even as We are One."
By St. Nikolai Velimirovish
"That they all may be one, as Thou, Father, art in Me and I in Thee" (John 17:21).
Brethren, God's mercy is great. When a righteous man feels it, he weeps; but when a sinner feels it, he is ashamed.
By the mercy of God, we are cleansed, illumined, saved, adopted and united with God Himself. However, no one should construe that, by this unity with God, we become of the same Essence with God and equal to God. We will never be of one Essence with God, nor equal to God, in the way in which the Father, Son and Holy Spirit are of one Essence and equal in being.
"That they all may be one" the Savior says to His Father on behalf of His disciples, "as Thou, Father, art in Me and I in Thee", and here He is thinking of the unity of love and not of the unity of nature. From love flows mutual obedience, mutual help, mutual mercy, meekness, humility, goodness, good will and sacrifice.
And when the Lord says, "Be ye therefore perfect, even as your Father which is in heaven is perfect" (Matthew 5:48), He does not mean that men can be equal to God, but means to show them the supreme example of perfection in every good thing. For many teachers of men have pointed to examples of perfection in some thing or some person, but not in God. Often enough, they have taught men evil, and pointed to it as an example of perfection. That is why the Lord teaches men to take the Heavenly Father as an example of every perfection, and to labor and strive for that true perfection, and not some other.
By the grace of God, we are all adopted of God and become one in Christ Jesus (Galatians 3:28). However, we do not become gods; we do not become equal with the Persons of the Holy Trinity. Do not forget that it is said in the Scriptures: "The heavens are not clean in His sight" (Job 15:15). The majestic powers of the heavens are not even equal to Him, so what then of man? However, by the grace of God, and because of the sufferings of the Lord Jesus, the faithful are raised up into unity with God, in love and spirit. Therefore, let us make an effort to do the will of God, that we in truth may be raised up to such majestic heights.
O Lord Jesus Christ our God, Who art the God of every mercy and goodness; uphold us in Thy mercy to the end, and be not angry with us, but rather forgive us. To Thee be glory and praise forever. Amen.