http://revach.net/article.php?id=1061

Section: Moadim   Category: Yom Kippur
Rav Yisroel Salanter - Teshuva On The Same Aveiros Year, After Year, After Year....
The Rambam says that real teshuva means that Hashem, who knows the secrets of your heart, must testify that you will never return to the aveira again. That is a tall order! Most people would not even be comfortable testifying on themselves. How can we do teshuva if it requires a 100% guarantee on the future?

Reb Yisroel Sanlanter answers with a question on this Rambam. He asks why does he say, Hashem "who knows what is hidden in the heart". Shouldn't it say "who knows the future." Therefore he explains that the requirement of the Rambam is that a person must resolve honestly in the depths of his heart that will never do the aveira again. If he makes an honest resolution and really believes that he is so disgusted by the aveira that there is absolutely no possibility for him to slip yet again his teshuva is accepted. Even if he reverts to his old ways it does not negate the teshuva that he has done. That is why the Rambam says you need the testimony of Hashem who knows what thoughts lie in the heart of every man.