John Lightfoot on John 5:17

[My Father worketh hitherto.] Our Saviour being called before the Sanhedrim, 1, asserts the Messiah to be God: and, 2, that he himself is the Messiah. 'The Son of God' and 'the Messiah' are convertible terms, which the Jews deny not; and yet have very wrong conceptions about 'filiation,' or being made a son.

St. Peter confesseth, Matthew 16:16, "Thou art the Christ, the Son of the living God." So also Caiaphas in his interrogatory, Matthew 26:63, "Tell us whether thou be the Christ, the Son of God?" But they hardly agree in the same sense and notion of sonship. Aben Ezra upon Psalm 2:12, Kiss the Son, confesseth that this is properly spoken of the Messiah; but in Midras Tillin there is a vehement dispute against true filiation. The same Aben Ezra likewise confesseth, that in Daniel 3:25, one like the Son of God is to be taken in the same sense with that of Proverbs 31:2, What, my son? and what, the son of my womb? But Saadias and R. Solomon understand it of an angel.

"There is one who hath neither son nor brother; the Holy Blessed; who hath neither brother nor son: he hath no brother, how should he have a son? only that God loved Israel, and so called them his children."

It is not unknown with what obstinacy the Jews deny the Godhead of the Messiah. Whence the apostle, writing to the Hebrews, lays this down as his first foundation of discourse, That the Messiah is truly God, Hebrews 1. Which they, being ignorant of the great mystery of the Trinity, deny; fearing lest, if they should acknowledge Messiah to be God, they should acknowledge more Gods than one. Hence they every day repeated in the recitals of their phylacteries, "Hear, O Israel: The Lord our God is one Lord." And so, being blind as to the mystery of the Trinity, are the more hardened to deny that.

Our Saviour strenuously asserts here the Godhead of the Son, or Messiah; namely, that he hath the same power with the Father, the same honour due to him as to the Father, that he hath all things in common with the Father. And hence he makes this reply upon them about healing on the sabbath; "My Father worketh on the sabbath day, so do I also."