Perhaps this euphoria or state of perceived security (if you like) that the Jews enjoyed might have continued had some of their own hierarchy not abjured the exclusivity of their own faith.
We find that suddenly and unexpectedly huge numbers of Jewish scholars and 'holy men' entered into the fold of Islam; Priests and Rabbis seriously began to look at what Muhammed (pbuh) was offering - besides prosperity - and wondered if they were doing the right thing in allowing the Prophet (pbuh) so much freedom in making so many inroads into their lives. Even some Rabbis were starting to look favorably towards the 'Religion'.
One such Rabbi, Abdullah ibn Salam, approached the Prophet (pbuh) and announced that he and his family had embraced Islam. His entire household! he said.
He then proceeded to proclaim the 'Oneness of Allah' and the articles of faith that are customary to the 'Acceptance of the Religion', and the Prophet (pbuh) welcomed him and his whole family into the religion of Islam.
Abdullah ibn Salam then said, 'O, Messenger of Allah, I fear for the well-being of my family and myself, should the others find out about our conversion.' He stared hesitantly at the Prophet (pbuh) who looked at him reassuringly.
'What is it you would like me to do?' the Prophet (pbuh) enquired.
'I would like you to ask them what would they do if I, Abdullah ibn Salam, were to accept Islam.' And he looked down at his feet, feeling awkward and not knowing what to say further.
The Prophet (pbuh) then placed his hand on Abdullah's shoulder and said. 'O, Abdullah...It is only Allah who gives guidance, and if He has so willed that you shall become a Muslim then have no fear. Allah will protect you!'
But when Muhammed (pbuh) approached the 'Jewish Council' as Abdullah ibn Salam had requested, he was told in no uncertain terms: 'Abdullah ibn Salam is our master and the son of our Master. He is a learned scholar and Rabbi, and he shall never denounce his faith!'
Thus when Abdullah ibn Salam went back to his people and told them about his conversion, they attacked him and spread all sorts of malicious gossip about him and his family to all and sundry in the Jewish quarters of Medina.
This incident also triggered the Jews' mistrust of Muhammed (pbuh) and their denial of him as a prophet. It also opened the path to all those enemies of Islam to form a bond with the Jews and systematically begin to undermine Islam.