Mmmh, let's say you have a master powerful enough to rival a God, would you lie to him?
I sure won't. He's a good master and all, but we don't know how he'll react when really pissed off. I don't mean he will personally lash out, I mean that the mana that he already has trouble controlling might do bad things if he gets angry.
And that's just plain stupid, too. I think the best way to protect him is to keep him well informed. The guy is getting more and more naïve, not because he's dumb, but because a ton of things are hidden from him.
Damn, when you think about it, it happened to him in our world too. His women friends kept their feelings hidden for a real long time... then they get angry when he misinterpret their action, based on what he actually knows.
Holy crap, I just realised that all the people he really trust are actually lying to him (Rembrant, Mio and Tomoe). If I'm the guy, and I realise that fact, I might grow to never ever trust anyone in this world anymore.
Instead of a God, he might actually become a real Maou.