What is a church? It is an assembly; and a Christian church is an assembly of faithful men: of men who know the truth, believe it, avow it, and adhere to it. The Greek word signifies an assembly summoned out of the whole population to exercise the right of citizenship. An ecclesia, or church, is not a mob, nor a disorderly gathering rushing together without end or purpose, but a regular assembly of people called out by grace, and gathered together by the Holy Spirit. Those people make up the assembly of the living God.
In order to have a church there must be a selection and a calling out; and that calling must come from God, who alone can call effectually. Concerning all the members of this select assembly there is an eternal purpose which is the original reason of their being called, and to each of them there is an effectual calling whereby they actually gather into the church…May God grant that our churches may rise to their duty, however painful it may be. Yes, may they keep close to the faith, for they cannot otherwise be the pillar and ground of the truth. An unholy, unregenerated church can never be the pillar of the truth. If there is a failure in vital godliness, if humble walking with God is neglected, then the church cannot long remain a healthy church of God.
Charles H. Spurgeon, 1878