From December 23, 2003:
While the word ἴσος can mean "similar" it can also have the denotation "equal." Why would the first-century Jews have been so upset, if they only thought Jesus was making himself like or similar, but not equal to his Father? Furthermore,
remember what Paul wrote in Phil. 2:6:
ὃς ἐν μορφῇ Θεοῦ ὑπάρχων οὐχ ἁρπαγμὸν ἡγήσατο τὸ εἶναι ἴσα Θεῷ
On the other hand, I find the words of Trinitarian scholar G.R.B. Murray of interest:
"Bultmann, however, went on to point out that the Jews failed to grasp that Jesus is the Revealer; second, they made the mistake of viewing equality with God as independence from God, whereas for Jesus it meant total dependence on God ([Bultmann] 244). In light of these (undoubtedly correct) observations, the expression 'equal to God' is a misleading interpretation of the declaration of Jesus. That Jesus spoke of God as his own Father rightly points to the unique relation to God, and it is the Evangelist's concern to make plain the nature of that relationship. But in vv. 19-30 we see a twofold emphasis that exists in tension: on the one hand there is the acknowledgement by Jesus of the total dependence of the Son on the Father, and on the other a consciousness of the Father's appointment of the Son to perform on his behalf works that God alone has the right and power to execute (vv 19-20, 21, 22, 26-27,30). It is perhaps not irrelevant to note that the Jews were ready, when they wished, to recognize that in certain conditions men could be spoken of as God. For example they viewed Ps 82:6, 'I said you are gods, sons of the Most High all of you,' as relating to the people of Israel. And they glorified in the fact that in Exod 7:1 God states that he has made Moses as God to Pharaoh, whereas since Pharaoh made himself as God he had to learn that he was nothing (Tanh. B sec. 12 in Str-B 2:462-64). It would seem that in their eyes God could exalt a man to be as God, but whoever made himself as God called down divine retribution on himself. They saw Jesus in the latter category" (John, page 75).
While I do not agree with Murray's comments in toto, I think the last few sentences of the quote provided above shed light on monotheism in ancient Judaism. Having said the foregoing, I would argue that certain Jews thought Jesus was making himself equal to God (not merely similar), but they were mistaken.