Sometimes a person feels low and Emunah seems out of reach. In such a state, the Yetzer Hara can say to a person, "What use is it for you to be strong and hold yourself back from falling into sin, you anyway feel so far from G-d and you anyway don't have now any Emunah or purity of heart!"
The Ba'al Hasulam, a great Kabbalist, was famous for trying to teach people how to serve Hashem Lishma (with the purest of intentions). However, he once said that "as great as we can imagine it is to serve G-d Lishma, the doing of Torah and Mitzvos without ANY pure intentions whatsoever is still a lot greater!" In other words, no matter how far our imagination can stretch to visualize the greatness of Lishma and how precious it is in G-d's eyes, - Sheloh Lishma, i.e. doing the Mitzvos even without any Emunah, - is greater - in G-d's Eyes - than that still.
What is reason? The rationale behind this is that G-d has great pleasure when "work" (yegiyah) is exerted by a human being to do his Mitzvos, even without any thought or belief.
So don't let the Yetzer Hara fool you and tell you that staying holy is worthless because your thoughts are anyway impure and your Emunah is weak! For it is more precious to G-d for you to do his Mitzvos than you can possibly imagine, even if you do them without any Emunah at all!