Adam HaRishon was hitting himself on his face, and he said, 'So great is the power of teshuvah, and I didn't know.'
We have to understand what Kayin meant when he said ונפשרתי, I made a compromise. What compromise did he make? And also, didn't Adam HaRishon know the strength of teshuvah? What did he learn from Kayin's teshuvah that he didn't know before?
The Chasam Sofer (Toras Moshe, Shabbos Shuvah) explains that Kayin didn't do teshuvah with a whole heart. Chazal tell us that Kayin was בוראו את ומרמה העליונה דעת שגונב כמי, he acted like someone who thinks he can outsmart and trick Hashem. (See also Rashi Bereishis 3:16). This means when Kayin said that he regrets his aveirah, he didn't mean it. His teshuvah wasn't sincere. Nevertheless, his teshuvah was accepted to the extent that he made a פשרה, a compromise with Hashem, and his aveirah was forgiven, at least partially. Adam HaRishon was amazed at the strength of teshuvah, and he said, "I didn't realize the extent of the power of teshuvah! Even when one's teshuvah is dishonest, it is sufficient to make a compromise."
Many times, the yetzer hara weakens a person's resolve to do teshuvah. The yetzer hara says, "You aren't doing teshuvah with a full heart. You yourself know that you will go back to your old ways, and you won't remain with your resolve. Why should your teshuvah be accepted?!" The person can reply, "Our Father in heaven wants our teshuvah, even when it is at a very low level. Even when we tell Hashem that we will improve our ways, and it is essentially רמיה, trickery and dishonesty, because it is likely we won't improve our ways, nevertheless, we are attempting to become better, and that is also precious in Hashem's eyes."
