The New City Catechism Question # 48: What is the Church?

God chooses and preserves for himself a community elected for eternal life and united by faith, who love, follow, learn from, and worship God together. God sends out this community to proclaim the gospel and prefigure Christ's kingdom by the quality of their life together and their love for one another.

2 Thessalonians 2:13
But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth.