The passage you cite is from Jesus’ commission to His twelve disciples (Matthew 10:23), when He sent them out to preach. In this commission He told them where to go, and what they could expect. Among the expectations was a warning of the persecution they would encounter, and in verse 23 a projection of their degree of success.
The meaning of the passage is clearer in the Lamsa Translation, which reads as follows: “When they persecute you in one city, flee to another; for truly I say to you that you shall not finish converting all the cities of the house of Israel, until the Son of Man returns.” In other words, the world as a whole will not be converted until Jesus returns and sets up His Kingdom. The disciples could expect at best to do only a measure of good. They would not be able to convert whole communities.
Not “till the Son of Man be come” the second time, not until Jesus returns will the task of teaching the gospel be completed. Only when the ultimatum “Fear God” is proclaimed worldwide (Rev. 14:7), when God’s judgments are in the world (Isa. 26:9) and the inhabitants of all lands are compelled to learn righteousness and all “know the Lord…from the least of them to the greatest of them” (Jer. 31:34)–only then will “all the cities of the house of Israel” be turned to God.