The Fullness of the Gentiles

Concise Bible Dictionary:

This is spoken of in connection with the olive-tree as God’s tree of promise and privilege on the earth. Israel was that tree (Jer.h 11:16); but because of unbelief some (perhaps the most) of the branches were broken off, and some Gentiles were, contrary to nature, grafted into the good olive-tree. But if these Gentiles do not continue in the goodness of God, they also will be broken off, the fullness of the Gentiles will have come in, and Israel will be reinstated (Rom. 11:15-25). It therefore follows that as the admission of the Gentiles to privilege is linked with the failure of Israel, so the taking up of the Jews again for blessing is linked with the apostasy of the Gentiles. A solemn fact for Christendom!

From Anstey’s Doctrinal Definitions:

This expression refers to the full number of elect Gentiles—whom God has chosen to be part of the Church—being saved (Rom. 11:25). God is presently conducting an outreach through the gospel in visiting “the Gentiles to take out of them a people for His name” (Acts 15:14). When the complete number of Gentiles whom He has chosen believe, the Lord will come and take the Church home to heaven in a glorified state (1 Thess. 4:15-18).