By The Church began following the conversion of many on the day of Pentecost (Acts 2:41-42) and will continue until Jesus returns for His Church (1 Thess 4:17). All Christians are included in the Church by the Holy Spirit (1 Cor 12:12-13) and gifted by the Holy Spirit for the mutual spiritual benefit of all (1 Cor 12:7).
The foundation of the church is the Lord Jesus Christ and the apostles and prophets of the NT era (Eph 2:19-20). Christ has gifted the Church with the NT apostles and prophets, and since then with missionaries to share the Gospel where each local church is then gifted with pastors and teachers to equip the believers to build one another up in the faith (Eph 4:11-16).
Christ is the Shepherd of the church with elders appointed to functions as under-shepherds of Christ in each local church (1 Pet 5:2-4).
The Church is kept pure by the mutual accountability by each in the church and by the church as a whole (Matt 18:15-17).
The primary activities of the Church is focussed on the building up of the saints (Col 1:28), although that does not exclude the outward focus of the Church through personal evangelism (Matt 28:19-20) and missionary support (Acts 13:1-3).
We are obliged to meet with the church to encourage one another to love and obedience, and excel in it until Jesus returns (Heb 10:24-25).