The verse does not say, "God chose us before the foundation of the world...." It says, "God chose us IN HIM..." The verse also doesn't say, "God chose us TO BE in him before the foundation of the world...." It says God chose US IN HIM... TO BE holy and blameless before Him...." The verse says nothing about God choosing who would become a Christian. The verse tells us what God chose Christians to be - holy and blameless. It never says anything about God choosing who those Christians would be. Yet people read the verse as if it says something it never says.

Basic Reading Comprehension:
The verse doesn't say God chose people to be Christians.
But it does say what God chose Christians to be.