In 1 Corinthians 5, we encounter a vision of the church not as one voluntary association among many [the denominational/independent view of the Church popular today] but as the covenant people of God; to be inside this community is to find life, and to be outside is to be in the realm of death.

Richard B. Hays, First Corinthians: Interpretation,
A Bible Commentary for Teaching and Preaching

The Theological Dictionary of the New Testament says metanoia [“repentance”] demands an “unconditional turning from all that is against God” and an “unconditional turning to God.”

Ronald Snyder, The Scandal of the Evangelical Conscience

…we cannot escape the fact that when people in the New Testament believed, they were added to churches (Acts 2:41,47, 5:14; 11:21-26; 14:21-23). This was not optional; it was inevitable.… those who have declared members of Christ’s body must pursue membership with an actual group of Christians, a visible local church. If you won’t pursue (local and visible) membership, are you really a (universal and invisible) member? Who’s to say?”

Michael Lawrence, Conversion

Instruction did not stop with groups for beginners in the early church!  There was teaching on false doctrine, and you see the heart of it in 2 Peter and Jude.  There was an agreed approach to ethics, and you see the essence of it in Ephesians and Colossians.  We, too, ought to be training people in these areas.  We should also train folks for marriage, in a day when one out of two marriages in the West collapses, and when Christian divorce rates are not much better than those of non-Christians.  We need to train parents for family life and solidarity in a day when the family is falling apart.  … The modern church seems to have forgotten its calling to train disciples.  Not so the early Christians.

Michael Green,  
Thirty Years that Changed the World: The Book of Acts for Today