According to the Holy Scriptures the unity of the Churches is the holy mystic tie of those who believe in Christ by the common confession of faith, hope and love for Christ, the Groom and by His common worshiping. In the Holy Scripture we find the character of this unity.
In the Prayer of our Savior to His heavenly Father is shown the unity of the Church. Before going to suffer His crucifixion, Jesus raises a fervent pleading prayer to His Father and asks Him to protect all those who believe and will believe in Him and keep them in the tie of peace:
Sanctify them by the truth; your word is truth. 18 As you sent me into the world, I have sent them into the world. 19 For them I sanctify myself, that they too may be truly sanctified.
20 “My prayer is not for them alone. I pray also for those who will believe in me through their message, 21 that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. 22 I have given them the glory that you gave me, that they may be one as we are one— 23 I in them and you in me—so that they may be brought to complete unity. Then the world will know that you sent me and have loved them even as you have loved me.
24 “Father, I want those you have given me to be with me where I am, and to see my glory, the glory you have given me because you loved me before the creation of the world.
25 “Righteous Father, though the world does not know you, I know you, and they know that you have sent me. 26 I have made you[b] known to them, and will continue to make you known in order that the love you have for me may be in them and that I myself may be in them.”
(John 17, 17-26).
So the unity of the Church stays in the unity of its members with Christ.
All those who believed in Christ through the Holy Apostles united themselves with Jesus and sanctified themselves in the Truth of God Father. Thus the union is internal, mystic, direct, divine, perfect, achieved by goodwill and divine love and nobody connects by an exterior tie to create unity. Those who believed had received the grace, the truth, the light and life through Jesus Christ and united themselves with Him. What can separate them of their union with Christ? If this tie is perfect what’s the use of another tie, of another faith? Those who believed were drawn to the Savior by the Father, Who sent Him (John 6,44) and they received the grace of redemption. And if the truth freed them from the slavery of sin who can deprive them of the freedom in Christ?
Those who believed became sons of light and partakers of the eternal glory so who can take away the enlightenment and glory from them? Those who believed were adopted by God through our Lord Jesus Christ so who can negate or annul this adoption? Those who believed became by the divine Communion partakers of the Body and Blood of the Lord remaining in Christ and Christ remaining in them so who can break these divine ties of unity?
Those who believe receive the Holy Spirit who is always the foundation of the church and presented it as being One, Holy and Catholic. So who can break this unity? It’s in vain any claim of an exterior tie and of another faith, especially of one that is rejected by the Holy Scripture, for ensuring the salvation of those who believe in our Lord Jesus Christ.
Excerpt from the book Saint Nektarios of Egina – Why the Pope and his followers separated from the Church of Christ, Evanghelismos, 2011