Q. How can the rapture of the church occur when the fullness of the gentiles come in when millions of gentiles will come to faith during the tribulation?
A. In his phrase “fullness of the Gentiles” Paul was referring to the Church. We know this because he said after this group comes in (is raptured), God will turn to Israel again. This is a reference to Israel fulfilling their final 7 years from Daniel 9:24-27. The millions of gentiles who come to faith during those final years are not part of the Church but are of a group we call Tribulation believers. Many will be martyred for their faith (Rev. 6:9-11, Rev. 7:9-17, Rev. 20:4) but those who survive will help repopulate the Millennium on Earth. They are the sheep of the Sheep and Goat judgment (Matt. 25:31-40)