That means ANY... sort of argument presented by those on man's pre-trib rapture theory falls flat, simply because Lord Jesus revealed His coming to gather His Church is AFTER the tribulation. The pre-trib theory instead falsely claims Jesus comes to rapture His Church PRIOR to the tribulation; Jesus said just the opposite.
Also, the Matthew 24:31 and Mark 13:27 verses directly parallel what Apostle Paul taught in 1 Thessalonians 4:13-17 about Christ's coming to gather His Church.
The nation of Israel is not a large enough for over a billion Christians.
The gathering of the elect in Matthew 24:31 and Mark 13:27 is referring to the final gathering of the children of Israel, the Jews, out of the nations to the land of Israel. Fulfilling a promise that God made in Deuteronomy 30:1-6.
1 And it shall come to pass, when all these things are come upon thee, the blessing and the curse, which I have set before thee, and thou shalt call
them to mind among all the nations, whither the LORD thy God hath driven thee,
2 And shalt return unto the LORD thy God, and shalt obey his voice according to all that I command thee this day, thou and thy children, with all thine heart, and with all thy soul;
3 That then the LORD thy God will turn thy captivity, and have compassion upon thee,
and will return and gather thee from all the nations, whither the LORD thy God hath scattered thee.
4
If any of thine be driven out unto the outmost parts of heaven, from thence will the LORD thy God gather thee, and from thence will he fetch thee:
5
And the LORD thy God will bring thee into the land which thy fathers possessed, and thou shalt possess it; and he will do thee good, and multiply thee above thy fathers.
6 And the LORD thy God will circumcise thine heart, and the heart of thy seed, to love the LORD thy God with all thine heart, and with all thy soul, that thou mayest live.