While it's true that a good deal of MxPx's lyrics have nothing to do with Christianity, they are indeed a "Christian punk band" rather than "Christians in a punk band". One doesn't have to look very far to see evidence of this in their lyrics. Here are a few random selections...
I'm looking for a girl who writes me songs and talks to Jesus all day long...
It's true we are a brotherhood and we all know it. Bonded by the son of God is how we live...
There's one thing left to turn to; it's not a girl and it's not the world. When everyone has their own thing, he's into you and he loves you too. He's what keeps me going when i'm afraid any my beds not made...
Been told once, been told twice he just won't listen to advice. It's in God's book but he don't wanna look...
You can run jump sit or stand around but not for long unless you've found Jesus Christ, God, Son and Sacrifice. Believe in him, he'll open up your eyes...
You already found the book with all the answers to all your questions. So take a closer look...
Now, having said all this... I'm a big fan of the guys myself, even though I consider myself to be atheist.