[PetroniusMaximus]

"So whatever it was that caused God to allow you to sin, it must reside within God Himself, right?"

I can't help but believe that when you and I talk about things being "internal" or "external" to God we are a bit like two flies discussion the nature of the TV set we have landed o­n. In other words, we are discussing things way over out heads.

God does not plan or orchestrate sin. He does not cause people to sin. He does not tempt people to sin. He does not want people to sin. He has given humans a measure of free agency and their misuse of it is the main source of sin. Nevertheless, He foreknows knows what people will do and is able to use their evil for His good purpose.


"Was He not able to prevent you from sinning?"

Could He not have called 12 legions of angels? 

God is "able" to do many things that He does not do.

[Frumanchu]

Could He not have called 12 legions of angels?

God is "able" to do many things that He does not do

So then you recognize that if God is able to prevent you from sinning but freely chooses not to do so, then it necessarily follows that your sin IS in fact according to His will in some sense (as you put it, to use it according to His good purpose).