It is typical for many groups to claim without a shred of evidence that the Trinity is some kind of modern invention with no basis in the Gospel, and even less in the Old Testament. It is of course Jews that make this accusation more frequently, but Muslims, atheists, and non-trinitarian heretics also love to do the same.
This is a companion discussion topic for the original entry at https://legiochristi.com/p/595731c7-dd02-4a4b-a71b-f8f6f7e1b9ae/