I’ve blogged before about how a preposition can make an enormous amount of difference. None more so, I don’t think, than in this example.

‘I believe you’ seeks to assure someone that you think they’re telling the truth. It’s almost like letting them off the hook if their story or defence is flimsy and they’re clutching at straws. It’s hardly a glowing endorsement.

Contrast this with ‘I believe in you.’ This has an altogether more committed, almost spiritual tone of endorsement about it. You’re placing your trust and faith in another person, and perhaps this is the greatest compliment or statement of togetherness that you can pay  them.

It changes the verb, the sentence structure and the meaning completely.