Do you really believe that the custom-crafted God your dad believes in "caused the Big Bang to happen" just because we haven't figured out a better answer? Really? Is "I dunno maybe we'll find out someday" that hard to accept?
Are you aware that most of the Ten Commandments are about how to properly worship God -- specifically the ancient Hebrew God? "Kill, Steal, and Lie" are only 3/10. Did your dad cherry pick only those three then? Why even bother with a supreme cosmic being when "don't be a douchecanoe" covers literally everything your dad didn't delete from the Ten Commandments?
Christianity has been a dominant faith for 2000+ years now.
Appeal to popularity, irrelevant. People believed that volcanoes were gods for thousands of years. C'mon.
You can't say that the bible is just "A load of BS written by a crazy person 2000 years ago" because the bible wasn't written by just one person. It was written by dozens of people that claimed to know or have seen Jesus, or that claimed they bore witness to the other significant events of Biblical times.
Lies. The four gospels contain ZERO eyewitness accounts. The entire bible as you know it was voted on by committee in the Middle Ages. Prior to that, the Old Testament was retconned dozens of times during periods of political upheaval. Don't believe me, look it up.
His argument is that it has to be at least somewhat legitimate if so many people during the Biblical times saw these things and believed them.
Elvis Presley died in 1977. Do you care to guess how many people have seen him alive since? Millions. So does that mean you should go ahead and believe he's still alive? I mean, those stores have to be at least somewhat legitimate. Did you know that the first gospel was written 50 years after Jesus was said to have been crucified, but Elvis only died 38 years ago? Did you know that the fourth gospel was written 200 years after Jesus died?
And that since people have been believing in them for the past 2000 years, he thinks something's gotta be right about it.
It's comforting to think there's a magic daddy figure looking out for you. Humans like that thought. Humans like the thought of someone with authority forgiving them for the bad things they've done in their lives. Jesus is a very comforting myth, if you are able to disconnect yourself from the seven headed dragon stuff. (Also, humans were forced to worship Jesus at the point of a sword for thousands of years, so there's that. Conquered peoples were quick to bend the knee to the gods of their conquerors.)
But the argument my dad left me with leaves me in a position where I have no reason to be an atheist
Look dude, your dad said "meh, I choose to worship a God that looks like X, and it's different than most other peoples' interpretations of what God looks like" and you have "no reason to be an atheist" now? That's nuts. He just admitted to you that he personally edited the concept of God into one that he prefers and you can't think of a single good reason to doubt that this God exists in real life? C'mon man, that's crazy.