First entry into the church is...

Calvin's Institutes (1559)
Book 4 of 4 - External Means by which God Invites Us into the Society of Christ and Holds Us Therein

Chapter 1 - The True Church with which as Mother of All the Godly We Must Keep Unity

17-22 - Imperfect holiness doesn't justify schism, but gives opportunity for forgiveness

17. The church is holy, not in being perfect (which would "leave no church at all"), but in that the Lord is cleansing her (Eph 5:25-27). The Lord has always had His church, from the beginning with Adam till now - Ps 89:3-4.

18. The prophets did not "establish new churches for themselves," though they called Israel Sodom and Gomorrah - Isa 1:10.

19. Christ and the disciples did not separate from the temple, in spite of the impurity in the Saduccees and Pharisees. "Those who participated in these same rites with a clean conscience knew that they were not at all contaminated by association with the wicked?" Cyprian: "Let no one so claim for himself what is the Son's alone, that it is enought to winnow the chaff and threst the straw and by human judgment to separate out all the tares."

20. It is "devilish... to be cocksure of our perfection." Forgiveness of sins is the "first entry into the church." This is marked by baptism: "we are initiated into the society of the church by the sign of baptism, which teaches us that entrance into God's family is not open to us unless we first are cleansed of our filth by His goodness."

21. God's ongoing forgiveness of our continual is also the basis for our remaining in the church, preserved by God's grace.

22. Christ gave the keys of the church to the apostles (Matt 16:19; 18:18; John 20:23), so His people would believe their sins were forgiven - 2 Cor 5:18, 20. The church regularly, in public and private (Acts 20:20-21), brings to us outwardly the forgiveness of our sins in Christ. "For very many... need personal consolation." We are never in this life so holy as to stand before God without needing forgiveness. We can't enjoy that forgiveness rightly without being members of the church. The church offers it through preaching the Word and administering the sacraments. "Seek forgiveness of sins only where the Lord has placed it."