This was why the Jews were seeking all the more to kill him, because not only was he breaking the Sabbath, but he was even calling God his own Father, making himself equal with God. (John 5:18 ESV)
Question: Do Trinitarians agree that at this point in John's Gospel the Jews had adequate grounds to make the claim that Jesus was making Himself equal with God?
The statement made by John is obviously a prolepsis. At this point in the course of events, no one was trying to kill Jesus, nor had He made any statements about God being His Father. As John does elsewhere (for example John 2:19-22 and 7:37-39), he provides information to the reader which is not yet present in the event. Unlike other occasions, John does not add an explanation or take a position. He simply gives the reader advance notice of what is coming: the Jews will try to kill Jesus. He also gives reasons for this: He made Himself equal to God by His actions on the Sabbath and by calling God His own (ἴδιον) Father.
Therefore, at this point the Jews do not have adequate grounds for their claim. That is the reason John adds the prolepsis. What is fair to say is that John wants the reader to understand the current events are part of the sum total of the claims. In other words, the Jews understanding of Jesus claiming equality was not yet fully developed or made explicit (see John 10:22-31), yet what is taking place is one piece of evidence which is consistent with what the Jew's believe.
John's prolepsis functions as a witness regarding the question if Jesus said He was God. Clearly the Jews claim He did. They understood His actions and statements were making Himself equal to God, which to the monotheistic Jews, was a claim to be God, and was punishable by death.