Interesting. I took that phrase as indicating the exact opposite. It implies that you don't really believe the person, but are electing to act as if you do (and to inform them of that fact) -- which means there is a lack of basic trust. I think because it's such an odd thing to say, it's hard to know how to interpret it.

In the context of the rest of your post, it seems to me to indicate a lack of trust even more strongly. But perhaps that's just me, and others would take it differently.

Why not just use probationary periods, but not emphasize the reason to the applicants? Everyone already knows what probationary periods are for, after all.