A pseudonymous caliphal letter made scriptural-falsification arguments against Christianity

Evidence

The letter attributed to ‘Umar (in the version that has come down to us) seems in fact to have been written by a Muslim writer of the ninth century. It provides a good example of Muslim polemical views of Christianity as they had by then developed. The author argues that the Christians have falsified their scriptures; that Jesus had never claimed to be God, but rather a messenger sent by God who had foretold the coming of Muhammad; that Jesus taught monotheism and not Trinitarianism. For all these arguments, he cites biblical proof texts (with varying levels of accuracy). He attacks the cult of relics and the cult of the cross and images. — John V. Tolan, Saracens, 266

Categorization

• QUESTION: The ‘varying levels of accuracy’ in the biblical proof texts — worth tracing which citations were garbled, and from which Greek or Syriac source.