One more point on the Ramban I cited yesterday: Why is disbelief in the Divine origin of mitzvos, even if one is perfect in his/her observance, a more severe offence than violating mitzvos in practice? I think the answer is found in Rabeinu Yonah, Sharei Tshuvah 1:6, who quotes the same pasuk as the Ramban and also notes the Torah’s stress on “yakim…la’asos”, not just “ya’aseh”, i.e. acceptance and belief, not just performance. R' Yonah adds an analogy to explain the idea: if a slave tells his master that he is willing to obey all the master’s instructions except one, the yoke of servitude is broken and the slave’s actions are in reality obedience only to his own will. Acceptance of the Divine authorship of mitzvos is acceptance of servitute to G-d.
In other words (you Briskers will like this), mitzvah performance really consists of two components: 1) the performance of G-d’s will; 2) kabbalas ol – the demonstration that one is subservient to G-d’s will. Even if one performs mitzvos, if that performance is undertaken as an expression of one’s own desires and sense of right and wrong (I cited in the past opinions who hold that even mitzvos sichliyos should be performed because of G-d’s command and not simply because they are reasonable), one’s religious persona is lacking. Ana avda d’KB”H! An eved can make mistakes in carrying out service and at times disobey, but nonetheless remains an eved. One who refuses to accept the yoke of avdus is in a different category entirely.