A Reader Asks: [I thought the statement] "The principle of sufficient reason does not apply to God" [...] was true, that it does not, as we cannot explain His origins. [But you] stated that it was false. How is that? Can someone explain this to me?
Ite ad Thomam Replies: The principle of sufficient reason has no exceptions: absolutely EVERYTHING has a reason for its own existence. The reason for the existence of x can be either (a) something else, which is the cause of x, or (b) x itself, in which case x is not its own cause, but merely the reason of its own existence. The latter (b) is the case with God: God is the reason for His own existence. He exists because it is part of his nature to exist (hence He calls himself "I Am Who Am" in Ex. 3:14, and Aquinas calls Him the Ipsum Esse Per Se Subsistens).
Let me illustrate with an example: a rectangular cake has a reason for being rectangular, namely, it was made with a rectangular mold. The mold itself has a reason for being rectangular, namely, it was made with something else that was rectangular. Ultimately, everything rectangular has a reason for being rectangular, and the ultimate reason is that it is modeled on a rectangle. Now, what is the rectangle's reason for being rectangular? The rectangle is rectangular, not because of something else, or because of some cause that makes it rectangular; rather, the rectangle is rectangular simply because it is part of its nature to be so.