Flavius Josephus, The Wars of the Jews
William Whiston, A.M., Ed.

[208] Now it was John who, as we told you, ran away from Gischala, and was the occasion of all these being destroyed. He was a man of great craft, and bore about him in his soul a strong passion after tyranny, and at a distance was the adviser in these actions; and indeed at this time he pretended to be of the people's opinion, and went all about with Ananus when he consulted the great men every day, and in the night time also when he went round the watch; but he divulged their secrets to the zealots, and every thing that the people deliberated about was by his means known to their enemies, even before it had been well agreed upon by themselves. And by way of contrivance how he might not be brought into suspicion, he cultivated the greatest friendship possible with Ananus, and with the chief of the people; yet did this overdoing of his turn against him, for he flattered them so extravagantly, that he was but the more suspected; and his constant attendance every where, even when he was not invited to be present, made him strongly suspected of betraying their secrets to the enemy; for they plainly perceived that they understood all the resolutions taken against them at their consultations. Nor was there any one whom they had so much reason to suspect of that discovery as this John; yet was it not easy to get quit of him, so potent was he grown by his wicked practices. He was also supported by many of those eminent men, who were to be consulted upon all considerable affairs; it was therefore thought reasonable to oblige him to give them assurance of his good-will upon oath; accordingly John took such an oath readily, that he would be on the people's side, and would not betray any of their counsels or practices to their enemies, and would assist them in overthrowing those that attacked them, and that both by his hand and his advice. So Ananus and his party believed his oath, and did now receive him to their consultations without further suspicion; nay, so far did they believe him, that they sent him as their ambassador into the temple to the zealots, with proposals of accommodation; for they were very desirous to avoid the pollution of the temple as much as they possibly could, and that no one of their nation should be slain therein.