I believe in one, holy, catholic and apostolic Church. (Nicene Creed)
Christianity from the First Century held to some idea that becoming a Christian meant joining a people, a community, the Body of Christ. There was no such thing as being a Christian alone, or that faith was an individualized affair between a isolated person and God. Christianity implied love, being united to Christ, and sharing a faith with other believers. If one stood apart, it was only in the sense that one belonged to and committed oneself to a community which had a set of values reflecting God’s Kingdom which could put you at odds with the values of the ‘nation’ in which one lived. There was a loyalty questions because one claimed membership in a kingdom not of this world, even if one lived in a worldly kingdom.
“…The Church, which is an alternative community to the status-quo human communities committed to and governed by Caesar (and analogous rulers) and by values contrary to the gospel.” (Michael J. Gorman, Reading Paul, p 8)
St. Basil the Great (d. 379AD) who grew up in the Roman Empire as it was being Christianized, writes about the essential nature of the Church as community for each Christian.
“If anyone claims to be able to be completely self-sufficient, to be capable of reaching perfection without anyone else’s help, to succeed in plumbing the depths of Scripture entirely unaided, he is behaving just like someone trying to practice the trade of a carpenter without touching wood. The Apostle would say to such: ‘It is not the hearers of the Law who are righteous before God, but the doers of the Law who will be justified.’ (Rom. 2:13) Our Lord, in loving each human being right to the end, did not limit himself to teaching us in words. In order to give us an exact and telling example of humility in the perfection of love, he put on an apron and washed the disciples’ feet. So what about you, living entirely on your own? Whose feet will you wash? Whom will you follow to take the lowest place in humility? To whom will you offer brotherly service? How, in the home of a solitary, can you taste the joy that is evident where many live together?
The spiritual field of battle, the sure way of inner advancement, continual practice in the keeping of the commandments, this is what you will find in a community. It has the glory of God as its aim, in accordance with the word of the Lord Jesus: ‘Let your light so shine before your fellows that they may see your good works and give glory to your Father who is in heaven,’ (Matt. 5:16) What is more, community preserves that particular characteristic of the saints which is referred to in the Scriptures thus: ‘All who believed were together and had all things in common.’ (Acts 2:44) ‘The company of those who believed were of one heart and soul and no one said that any of the things which he possessed was his own, but they had everything in common. (Acts 4:32)[…] Whoever loves God loves his neighbor. The Lord says: ‘Anyone who loves me will keep my word’ (John 14:23) and ‘This is my commandment, that you love one another as I have loved you.’ (John 15:12) So then, whoever loves his neighbor loves God, and God reckons our unity with our brothers and sisters is unity with him. One is reminded of Moses. He loved his brothers and sisters so much that he asked to be blotted out of the book of the living if the people did not receive forgiveness of their sins. (Exod. 32:32).” ( Drinking from the Hidden Fountain, pp 215-216, 218)
Becoming and being a Christian means being part of the Body of Christ, it means being in relationship to Christ and all those who believe in Him. As St. Basil points out even Moses does not receive the Law apart from God’s people, but rather even portrays himself to God as being one with those people, unwilling to be saved apart from them.