In the third article of the Nicene Creed we confess that "we believe ... in the one holy catholic and apostolic Church, the communion of saints..."
The Church is not an organization, it is a Body, the Body of Christ. The Church is the fellowship of the saints, the communion of saints. She is not of this world, but belongs to and is from God, established by and in Christ. She is the full number of the elect from every tribe and nation of men.
The Church is founded on the apostles and prophets with Christ Jesus as the cornerstone. She is made up of living stones, Christians, fashioned together into the Temple of the Lord. The Church is the Temple of God where God dwells with man and man with God. Without the Church there is no salvation because it is through and in the Church that God works salvation for men.
Thus the Creed continues, "I acknowledge on Baptism for the remission of sins, and I look for the resurrection of the dead and the life of the world to come." The Church baptizes, remitting the sins of those she baptizes. The Church looks for the resurrection, which is Jesus, and waits for the life of the world to come.
A congregation is the local expression (for lack of a better word) of the Church catholic. In the congregation, as in the whole Church, Jesus is worshipped, the Word of God is read, the Sacraments are administered, and God's will for us is made known through the testament of Jesus and the proclamation of the Gospel. One cannot belong to the Church without having some tie to a congregation, according to Jesus' words to His apostles, "Wherever two or three of are gathered in my name, there I am among them" (Matt. 18:20). That doesn't mean that if you don't go to church or officially belong to a congregation you are not a Christian. But it does mean that if you are a Christian then you come from a congregation and are obligated to join with the community of saints in the gathering of Jesus. Being a Christian without being a part of a congregation is like being a son or daughter without a family, it makes no sense and cannot last long.
Joining a congregation is not saying that everything that pastor or congregation does is sacrosanct or of God. But we're not called to look for "perfect" congregations to join, we're called to be one Body. NOT joining a congregation that preaches the Word of God - however clumsily or elementary - and administers the sacraments - however haphazardly and awkward - is tantamount to saying that you bring God to the congregation rather than God bringing you to Himself through His Church, which is what the Scriptures say, "The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?" (1 Cor. 10:16)