For me, this has always been one of the toughest questions to answer.
But when a student asks me this and is really searching, I try and walk with them for a few weeks seeking with them an answer for this, not just giving them the answer, but hoping to guide them to discovering it on their own.
I will say right out of the gate that I believe the answer to this question is one that doesn’t make sense, and requires a great deal of Faith. But in my mind, if it didn’t require Faith and it was completely logical, there is no way it can be true, because God is Infinite and we are finite.
For me, the biggest hurdle to answering this question is breaking down the pre-conceived notion that most students come into a discussion like this with, and that is the idea that for Every Thing, there must be a cause. The deductive reasoning here argues that if God exists, he must have at one point NOT existed.
So if I sat down with a Student and he asked me this question, my simple Answer would be
“God is not like You and I. What I am about to tell you may make absolutely no sense to you, but its what I fully believe. Call me ignorant, but God has shown it to be true, and I don’t think I’m Crazy. You would probably tell me you believe this line of thinking, that for every action, there must be a cause, that essentially, for every thing that has happened, something has had to make it happen. With God, that simply isn’t the case. He exists, I know this to be true, but again, he is not anything like you and I, and because of this, there is nothing that says he Has to have a start like we do. I can’t tell you where God came from, and ultimately, I will never know the answer for myself. But that doesn’t make God less plausible to me, it just makes him more Amazing than any other “thing” in this universe.”