A couple of reasons, none of which I am convinced are enough for those who don't want to believe them, but that's the way of faith. [smile]
First, there is a big difference between believing in an impersonal beginning and claiming that a very Personal Being has always existed. The difference between impersonal matter+space+time always "being there" (actually, then, there WAS no actual "beginning" for the pantheist) and an actual "Person with personality" existing before matter, space and time, is a huge one.
Secondly, our belief that God is the eternally existent one answers the problem of "First Cause." Before anything happened or existed, something else had to be there, causing it to happen or exist. We live in a cause-and-effect universe. Eventually, if you go back far enough, you have to come to what philosophers call the "First Cause," an "uncaused cause," who (or that) simply WAS. If there was ever a time when God did not exist, then something or someone would have had to create Him. But that's not the way it is; God is the "the buck stops here" First Cause where everything starts, except for Himself. It's logical, but for those who don't want to accept their accountability to God, they can blow it off. It doesn't make it any less true, though.
Boy, you make me glad I'm "preaching to the choir!"
I hope this hasn't clouded the issue further. But thanks so much for writing!!