Thou art Christ the Son of God. The older authorities omit "Christ," and read simply, "Thou art the Son of God." For they knew that he was Christ; better rendered, that he was the Christ, or Messiah.
After the Crucifixion, but not till then, "Christ" became a proper name. It was before simply a title, signifying "the Messiah," "the Anointed One." These words of the evil spirits do not seem to have been prompted by any design, as some have supposed, to excite the people either for or against the fresh Teacher; they are simply a cry of involuntary adoration.
They knew who that poor Carpenter-Rabbi was; they had seen him in his Divine glory!