Albert Barnes on Matthew 27:54

Now when the centurion, etc. Centurion, a captain of a hundred soldiers. He was here placed over the band that attended the crucifixion.

They feared greatly. They regarded these things as proof that God was angry, and they were terrified at the prospect that vengeance was coming on them.

Truly this was the Son of God. They had heard, probably, that before Pilate he professed to be the Son of God. Seeing these wonders, they believed that he was true, and that God was now attesting the truth of his professions. The centurion was a heathen, and had probably no very distinct notions of the phrase the Son of God; perhaps understanding by it only that he was like the heathen heroes who had been deified; but he certainly regarded these wonders as proof that he was what he professed to be. In the original it is, "A son of a god;" an expression perfectly suitable to a polytheist, who believed in the existence of many gods. Mk 15:39 says, that they affirmed that "this man was the Son of God." Luke, Lk 23:47 that they said, "Certainly this was a righteous man." These things were said by different persons, or at different periods of his sufferings--one evangelist having recorded one saying, and another another.

(u) "Now when" Mk 15:39, Lk 23:47