Just listen to what people say about these guys (in the King's case, we can only really judge him by what we're told).
Yes, but her son [Orgon] is even worse deceived;
His folly must be seen to be believed.
In the late troubles, he played an able part
And served his king with wise and loyal heart,
But he's quite lost his senses since he fell
Beneath Tartuffe's infatuating spell.
We serve a Prince[the King] to whom all sham is hateful,
A Prince who sees into our inmost hearts,
And can't be fooled by any trickster's arts. (5.7.18-19)
When it comes down to it, both Orgon and the King are good, virtuous people; it just seems that Orgon's lie detector is not quite as effective. The King's keen insight highlights just how blind Orgon can be.