His saying, If ye believe not that I am, without adding any thing, proves a great deal. For thus it was that God spoke to Moses, I am that I am. But how do I understand, I am that I am, (Exod. 3) and, If ye believe not that I am? In this way. All excellence, of whatever kind, if it be mutable, cannot be said really to be, for there is no real to be, where there is a not to be. Analyze the idea of mutability, and you will find, was, and will be; contemplate God, and you will find, is, without possibility of a past. In order to be, thou must leave him behind thee. So then, If ye believe not that I am, means in fact, If ye believe not that I am God; this being the condition, on which we shall not die in our sins. God be thanked that He says, If ye believe not, not, If ye understand not; for who could understand this?
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tract. xxxviii. 8