(Teachings from Gita) Only if you believe in what you do can you be devoted to it and if you're completely devoted to what you do then you would be in total control of your mind and senses. If you've control over your mind and senses then you'll always be focused on your goal. For example, a businessman who's totally devoted to his business is not panged by hunger, doesn't care about day or night: as he has all his senses under control. If you believe in yourself you would not be possessed by the vices of jealousy, rivalry, peer pressure and such things. Faith or belief is very important. As an example, I always feel that for a doctor's prescription to cure you it is essential that you believe in that doctor. You can win a losing battle, if you have faith. From faith comes devotion and from devotion, a control over the senses; and one who overpowers his senses can do anything. This brings to my mind the dialogue between Siddhartha and Kamaswami.
A man who lacks faith and wisdom cannot be happy in either worlds. He'd always be shrouded by doubt and dilemma; if he can't decide then he can't achieve because he's not sure what he has to achieve. He can't eliminate the doubt because he's neither wise enough to think nor believes in what he's doubting (hence, the doubt).