Juan 5:18

For this cause therefore the Jews sought the more to kill him, because he not only broke the sabbath, but also called God his own Father, making himself equal to God.

He not only broke the sabbath ... is the allegation of the priests, not the statement of the apostle John. See under preceding verse.

Making himself equal to God ... How strange it is that some can read the New Testament and then deny that Jesus claimed to be God. Even his enemies knew full well the implication of his words. Also, it was exactly upon this claim, which they construed as blasphemy, that they based their demands of Pilate that he be crucified (John 19:7). These two verses (John 5:17-18) are among the most important in Scripture, especially as they relate to the heresy of Arius (died 336 A.D.) and Sabellius (circa 230 A.D.), the former teaching that Christ was a created being, and the latter affirming that God, the Holy Spirit, and Christ are identical, and that Jesus was not God come in the flesh. As Trench wrote:

Other passages may contain as important witness against Arian, other against the Sabellian, departure from the truth; but this upon both sides plants the pillars of the faith.[9]

This open break between Jesus and the ruling hierarchy was sharp and irrevocable; and, fittingly, Jesus spoke upon this occasion at some length to his enemies in a vain effort to persuade them of the truth of his words and of his claim to be the Messiah. The rest of this chapter is taken up with this overwhelming testimony of the Lord Jesus concerning himself.