Just as it is impossible to accomplish anything without will, or to practice a discipline on a constant basis without decisiveness, it is equally impossible to ascend spiritually without having faith in oneself and in the power of one's devotions.
That is to say, we have already established the fact that doing good deeds is not sufficient in and of itself; a person must himself become good. He must take his character traits and transform them to goodness; just as Israel as a whole is engaged in a process of ascent and of growth, so must he be. But it is very difficult for a person who is in a lowly state, who feels that his personality is composed of degraded traits and that even his essence is ignoble to believe that his character will be transformed and made virtuous, and that he himself will experience spiritual ascents.
Because of this, he will be halfhearted and will not make a real effort. He will perform good deeds, but that is all. He will not try to strengthen himself in order to transform his being itself into good.
And yet, have we not said over and over again that you do not know yourself, that all you see is the outer covering - what is inside is concealed from you as well? Why doubt your own ability, thus destroying your own great future, when you do not really see and cannot really know?
Desire, decide, and believe in the power of Israel that is within you and within your devotions. Then you will see whether or not you will become holy as others have, and whether or not you too will shine like a bright star...*There are so many valuable things to take away from this small excerpt.
First of all, the Rebbe is teaching us an important concept of avodah: there are many levels to serving God, but if one wants to truly grow, he has to internalize his dealings on a "molecular" level. When performing a mitzvah, one has to do it with the intent and the desire that it should have an impression on the very fiber of his being, refining him, altering his spiritual construction; it doesn't happen without a cognizant effort.
But most important, Reb Kalonymos Kalman once again begs us to focus on our innate qualities. This element of faith in oneself does not stem from hubris but rather from knowledge of a special power vested within us by HaShem; as a member of Knesset Yisrael we have to ability to tap into the deepest reservoirs, the richest potential that manifests itself in each Jew through his or her unique expression. It is rooted so deep that we cannot even see it without consciously focusing on that power, and striving to coax it out.
* translation adapted from A Student's Obligation by Rabbi Micha Odenheimer