In a person's service of God, what's more important: thoughts, words, or actions? Most people would probably debate between words and actions. Words, after all, carry great meaning, and can affect others around us profoundly. Our actions directly affect ourselves and our neighbors, both for better and for worse. But thoughts remain private, intimate and personal. While our thoughts carry great importance for us, how do they relate to our worship of God?
Ibn Ezra (on 20:1) suggests that of the three categories, our thoughts are by far the most important, and that the Torah gives numerous commandments to direct our thoughts in a religious manner, including, "and you shall love Hashem your God", (in the Shema), "you shall fear God", "you shall love your fellow man as yourself," and many others. In addition, the Torah conveys negative thought commandments as well, prohibiting us from hating our fellow man, taking vengeance, or even entertaining thoughts of idolatry.
According to Ibn Ezra, our thoughts lead the way for us to follow. We control them -- and not the other way around. For this reason, the Torah commands us to direct our thoughts in positive, constructive ways demanded by the Torah, and in that way our words and actions will follow suit.
For this reason, the very first commandment deals with thoughts and belief: "I am the Lord Your God." If we believe and accept this fact in our hearts, then all the other commandments become that much easier to accept and fulfill. But without it, the other nine commandments don't mean very much at all.
One other thing crossed my mind. Following Ibn Ezra's rule about the critical nature of our thoughts, it's important for us to not only entertain and maintain a belief in God and faithful thoughts, but that we also keep our thoughts positive and energetic. When a person fills his thoughts with positive energy, those positive thoughts have the power to carry him or her a long, long way.