Albert Barnes Commentary Matthew 10:23

"But when they persecute you in this city, flee into the next: for verily I say unto you, Ye shall not have gone through the cities of Israel, till the Son of man be come." — Matthew 10:23 (ASV)

When they persecute, etc. They were not permitted to throw away their lives. Where they could preserve them, without denying their Lord, they were to do it. Yet all the commands of Christ, as well as their conduct, show that they were rather to lay down their lives than deny their Saviour. We are to preserve our lives by all proper means; but rather die than save ourselves by doing anything wrong.

You shall not have gone over the cities of Israel, etc. That is, in fleeing from persecutors, from one city to another, you shall not have gone to every city in Judea, till the destruction of Jerusalem, and the end of the Jewish economy. See Barnes on Matthew 24:28-30.

By the coming of the Son of man, that is, of Christ, is probably meant the destruction of Jerusalem, which happened about thirty years after this was spoken. The words are often used in this sense. See Matthew 24:30; Mark 13:26; Luke 21:27–32.