To say they were "trodden underfoot" is descriptive. The native populations were destroyed. They were conquered. They died. Their remains returned to the earth upon which the Gentiles trod.
You must keep this image in mind as you read about the future of the Gentiles being trodden underfoot. We will get to that later in this same prophecy by Christ.
What is meant then by saying "they are of Christ" on the one hand, and saying, "receive not the Gospel, neither the testimony of Jesus?" on the other. How do you reconcile these two things? One damns to the Telestial Kingdom, the other exalts.