Pastor Greg Nichols recently delivered thirty lectures on the doctrine of the church for Reformed Baptist Seminary. Several of his introductory lectures include a detailed exegesis and analysis of the 113 NT passages where ἐκκλησία (ekklesia) the primary Greek term for the church, occurs. Then he devotes an entire lecture to collating the data of the analysis and developing a definition of the church that is Reformed, Baptist, and (most importantly) biblical. Much of the rest of the course expands on and expounds the various facets of this definition.

One Definition in Five Forms

For pedagogical purposes, he offers his definition in five distinct forms: (1) A 17th Century Confession Version, (2) A Modern Update Version, (3) A Shorter Catechism Version, (4) An Abridged Modern Version, and (5) A Pocket Version. For brevity's sake, I'll only site the "Shorter Catechism" version below. But I'd highly commend to you the entire video lecture where you'll find the other four.

The church is Christ's saved society: purposed in God's eternal plan and solemn pledge of salvation, portrayed in covenant promises of salvation, formed in Christ's accomplishment of salvation, preserved through the gospel application of salvation, and glorified in the completion of salvation.