The unity of all Christians

The glory which You have given Me I have given to them, that they may be one, just as We are one; I in them and You in Me, that they may be perfected in unity, so that the world may know that You sent Me, and loved them, even as You have loved Me. (John 17:22-23)

The unity arises from God's nature. The Trinity - the persons of God are inseparably, completely one. In Jesus the Word became man and he maintained perfect unity with the Father till the end through his obedience, in which he is an example for his disciples. Just as Jesus lived in a perfect spiritual, volitional unity, the disciples too are called to live in spiritual, volitional, attitudinal unity with one another.
In his prayer, Jesus asks the Father that the unity among the disciples may be realised (John 17:11.17-24) also with those who will listen to their word. They are the Christians of every era (John 17:20-21). This perfect example shows that the depth of the unity among the disciples has no limit, for the example is the relationship between the Father and the Son. In the same way as the Father is one with the Son and the Son is one with the Father, and there is nothing that could separate them, the disciples can be also one through the Son.

This unity exists in reality as a sign for the world and for the people who seek the truth (John 17:21c.23b), and is not an unreachable ideal.

Jesus was aware of the fact that he prayed for sinful people. However, through the act of salvation the opportunity for real unity with each other arose for those who seek the deepest possible unity with the Father. The community with the living God forms the unity and the bonds of love by Christ and the Spirit. Unconditional trust, unlimited love and obedience, being ready for any sacrifice; these describe the relationship of the Son to the Father. This is the way for any follower of Christ in every time. In the repentance, God gives wish for giving all our life into His hands, yielding the leading to him and identifying with the will of the Father. According to the Bible to be one with Christ means also to be one with other Christians. (Acts 2:37-38)

So then, those who had received his word were baptized ... (Acts 2:41) Day by day continuing with one mind in the temple ... (Acts 2:46b) And the congregation of those who believed were of one heart and soul ... (Acts 4:32a)

Because these people seek the fellowship with God and other Christians, God can lead them together, even if they come from different backgrounds. God wants every man to repent and He has done and still does everything to enable that, but He knows that it must be the personal decision of His free-willed creatures.

WHAT DOES THE UNITY AMONG CHRISTIANS CONSIST OF?

1. Teaching

They were continually devoting themselves to the apostles' teaching ... (Acts 2:42)

... they received the word with great eagerness, examining the Scriptures daily ... (Acts 17:11b)

And he entered the synagogue and continued speaking out boldly for three months, reasoning and persuading {them} about the kingdom of God. But when some were becoming hardened and disobedient, speaking evil of the Way before the people, he withdrew from them and took away the disciples, reasoning daily in the school of Tyrannus. This took place for two years, so that all who lived in Asia heard the word of the Lord, both Jews and Greeks. (Acts 19:8-10)

In pointing out these things to the brethren, you will be a good servant of Christ Jesus, {constantly} nourished on the words of the faith and of the sound doctrine which you have been following. (1 Timothy 4:6)

You, however, continue in the things you have learned and become convinced of, knowing from whom you have learned {them,} and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus. All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work. (2 Timothy 3:14-17)

These passages illustrate the importance of getting to know the apostolic teaching, which stems from Christ. The Holy Spirit leads every Christian, and anyone who seeks God to this teaching.

... Because you have seen Me, have you believed? Blessed {are} they who did not see, and {yet} believed. (John 20:29)

But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. (John 16:13)

I will ask the Father, and He will give you another Helper, that He may be with you forever; {that is} the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him, {but} you know Him because He abides with you and will be in you. (John 14:16-17)

He is the one who enables the saved believers to reach unity in teaching, understanding and interpretation of the Word, for it is their desire to "worship the Father in spirit and truth". (John 4:23-24 and Philippians 2:1-2)
The idea that in essential questions (e.g. the salvation through Christ, predestination, papal primacy, infallibility of the pope, etc.) God's Spirit would lead open-hearted people who love the truth to different perceptions is ridiculous. (Ephesians 4:1-6) For He wants every Christian to...

... attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ. As a result, we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming; (Ephesians 4:13-14)

Diverging from the right teaching presented a real danger right from the beginning. Therefore Jesus and the apostles warned the Christians to be watchful and able to distinguish right from false teachings. (1 Corinthians 2:12-15)

But as for you, speak the things which are fitting for sound doctrine. (Titus 2:1)

O Timothy, guard what has been entrusted to you, avoiding worldly {and} empty chatter {and} the opposing arguments of what is falsely called "knowledge". (1 Timothy 6:20; The Greek word used here for knowledge is gnosis which is also the name of a false doctrine which Paul alludes to here.)

Now you followed my teaching, conduct, purpose, ... (2 Timothy 3:10a)

If anyone advocates a different doctrine and does not agree with sound words, those of our Lord Jesus Christ, and with the doctrine conforming to godliness, he is conceited {and} understands nothing ... (1 Timothy 6:3-4a)

... holding to a form of godliness, although they have denied its power; Avoid such men as these. (2 Timothy 3:5)

The recognition of the significance of unity can be found both in the Old Testament and among the ecclesiastical writings of the first centuries.

Behold, how good and how pleasant it is For brothers to dwell together in unity! It is like the precious oil upon the head, Coming down upon the beard, {Even} Aaron's beard, Coming down upon the edge of his robes. It is like the dew of Hermon Coming down upon the mountains of Zion; For there the LORD commanded the blessing--life forever. (Psalm 133)

Hear now then concerning the stones that are in the building. The square and white stones which agree exactly in their joints, are the apostles, and bishops, and doctors, and ministers, who through the mercy of God have come in, and governed, and taught and ministered holily and modestly to the elect of God, both they that have fallen asleep, and which yet remain; and have always agreed with them, and have had peace within themselves, and have heard each other. For which cause their joints exactly meet together in the building of the tower. (The Shepherd of Hermas - III. Vision 51-53)

2. The attitude, life-questions

...make my joy complete by being of the same mind, maintaining the same love, united in spirit, intent on one purpose. (Philippians 2:2)

Brethren, join in following my example, and observe those who walk according to the pattern you have in us. For many walk, of whom I often told you, and now tell you even weeping, {that they are} enemies of the cross of Christ. (Philippians 3:17-18)

Therefore I exhort you, be imitators of me. For this reason I have sent to you Timothy, who is my beloved and faithful child in the Lord, and he will remind you of my ways which are in Christ, just as I teach everywhere in every church. (1 Corinthians 4:16-17)

Therefore I, the prisoner of the Lord, implore you to walk in a manner worthy of the calling with which you have been called, with all humility and gentleness, with patience, showing tolerance for one another in love, being diligent to preserve the unity of the Spirit in the bond of peace. {There is} one body and one Spirit, just as also you were called in one hope of your calling; one Lord, one faith, one baptism, one God and Father of all who is over all and through all and in all. (Ephesians 4:1-6)

Now may the God who gives perseverance and encouragement grant you to be of the same mind with one another according to Christ Jesus, so that with one accord you may with one voice glorify the God and Father of our Lord Jesus Christ. (Romans 15:5-6)

That does not mean uniformity, it rather expresses that each of the brothers endeavour to build up the community, the body of Christ according to his own gifts and abilities. (1 Corinthians 12:4.13 and Ephesians 4:16) Each of the members is equally important. (1 Corinthians 12:14-27)

The Bible does not distinguish between theory and practice:

Now you followed my teaching, conduct, purpose, faith, patience, love, perseverance ...(2 Timothy 3:10)

Therefore be imitators of God, as beloved children; and walk in love, just as Christ also loved you and gave Himself up for us ... (Ephesians 5:1-2a)

By this all men will know that you are My disciples, if you have love for one another. (John 13:35)