"Therefore I say to you, the kingdom of God will be taken away from you, and given to a people producing its fruit."

Who it was taken from:

Jesus was speaking to the chief priests and Pharisees (see Matthew 21:45) – the religious leaders of first-century Israel. These leaders represented the nation as a whole, especially the unfaithful generation that rejected their Messiah. So, the kingdom was being taken from apostate Israel, particularly those who failed to recognize and receive Christ.

Who it was given to:

Jesus says it would be given to a people producing its fruit. This refers to the faithful remnant, both Jew and Gentile, who would form the New Covenant community. These are those in Christ, the ones Paul calls the true Israel of God (Galatians 6:16), or the holy nation and royal priesthood (1 Peter 2:9-10).

Summary:

Taken from: Unfaithful, Old Covenant Israel (especially its corrupt leaders)

Given to: The New Covenant body of believers (Jew and Gentile alike) who bear fruit through faith in Christ.

This was fulfilled in the first century when the kingdom transitioned at the end of the Old Covenant age (70 AD) to those who inherited the promises through Christ.