We always, infallibly, act according to what we believe. More specifically, we act according to what we believe will bring us the greatest happiness. If one is faced with the decision to be disciplined in his/her diet, or to have a slice of red velvet cake, then chooses the cake, it is because he/she believes that the cake will bring greater satisfaction than being disciplined.
But, what about will power? I have really wanted the cake and walked away even though I didn’t want to. Didn’t I act contrary to what I believed would bring me the greatest satisfaction? Actually, no. What motivated me to walk away? It was a future happiness attained by disciplined dieting. You see, the will is not where the power lies. The power lies in desire. What we truly desire most dictates the decisions of the will. And doctrine informs the source of desire.
Everybody is living according to a set of doctrines. Doctrine is simply a teaching. We have teachers all around us trying to instill in us a set of beliefs by which we will live. Advertisers try to teach us that satisfaction comes in buying their products. There are political teachers that vie for our allegiance to their particular worldview. Whatever teachers we buy into will determine what we desire, and as a result, what our wills pursue.
Christian doctrine, likewise, is about informing the source of our desires. The reason we need to study doctrine and theology is so that our desires will be transferred from temporary, inferior pleasures to the eternal and superior pleasures found in God. Sometimes we will get a taste of these pleasures in this life, but often they serve to keep us from eating the cake so that we will reap their benefits later.