When a person acknowledges (I’ve come to not like the term “accepting Christ as savior” because he is the Savior whether we accept him or not) Jesus as Messiah/Christ/King, he is not simply agreeing to allow Jesus to save him for a future in Heaven. We are being called into God’s creation restoration program. That is, God is restoring all of creation to its original beauty and more. God’s plan and example for how this should be done is found in his Church, the Bride of Christ. When we say “I do” to Christ, we are also and inextricably saying “I do” to the Church. You can not have Jesus, and not have his Church. It is in her that we are supposed to be able to find accountability, support, vision, compassion, and fellowship. It is in her we are supposed to find community.
The Christian existing outside the Church is not a fully realized Christian. It goes against the grain of our culture, but your walk with Christ is really not just between you and Him. We are not individuals who agree to certain terms of a contract that enables us to then work together for particular causes. We are many who are becoming one.
C.S. Lewis speaks of the doctrine of the Trinity as the ground for all Christian concepts of community. There is a selfless love within the Triune God where each member selflessly serves the other. The Son exalts and praises the Father, and defends the honor of God the Spirit when he warns the Pharisees of the unpardonable sin of blasphemy against the God the Spirit. The Father and the Spirit descend upon the Son at the baptism of Jesus, praising him and proclaiming his validity to the on lookers. The Father resurrected the Son, and has placed him on the throne of David in Heaven. All parts of the Trinity selflessly serve the other. The Three who are One is not graspable in human terms, yet He serves as the template for the Christian understanding of relationships. In Ephesians, Paul speaks of the many different people, the Jews who were close already to God, and the Gentiles who were far off, as being made into one new person. At its heart, the Epistle to the Romans is showing us how God is in fact making a new people out of two groups. Marriage is the two who become one flesh, and the family is made up of individuals who come together in love for one another to form that single unit.
Coming then full circle to the Church. The Christian who is not invested in the Church is a stunted Christian because she is missing out on one of the important desires of God for all of humanity: communities built on selfless love as we witness it within the Godhead Himself. I suppose there are people who have a vibrant relationship with God outside of the Church, but if they exist, they are the outliers, the anomalies. A Christian can’t make it on his own.