אלה הדברים אשר דבר משה These are the words that Moshe spoke
The Ramban in his introduction to Sefer Devarim explains that before Moshe transmitted the Torah to the Jewish people, he gave them rebuke. As he did this, he reviewed all of the places and incidents in the desert at which the Jewish people sinned, and how Hashem dealt with them in a merciful way. Moshe did this in order to encourage them so that they would realize that even when they sin, while HaShem may punish them, He is still always merciful. With this encouragement, the Jewish people would not be afraid to continue into Eretz Yisrael. They would realize that if they were to sin, Hashem would not wipe them out but, rather, He would give them a chance to repent and do tshuva.
Moshe was concerned that if the Jewish people believed that if they were to sin and would not have a chance to fix their ways, they would not even continue. He realized that the Jewish people themselves knew it was very likely that they were going to sin, and, therefore, they needed to know that there was a mechanism for atonement. We find this idea in the Gemara Chagiga, in which the Gemara relates the story of Eliysha Ben Avuya. When Eliysha ben Avuya sinned, he heard a heavenly voice that told him he was no longer able to repent. After hearing that voice, the Gemara tells us that even when he may have done tshuva, he didn’t do so because he felt that his situation was hopeless. It was for this reason that Moshe wanted to encourage the Jewish people to recognize that there is always a chance to repent and receive atonement for our misdeeds.
The Gemara in Taanis(20a) expounds on the Pasuk in Eichah(1:17) that compares Yerushalaim to a women who has become a niddah (ritually impure). The Gemara tells us the reason this imagery is used is to tell us that just as a women can become pure, so, too, Yerushalaim will return to its holy status as well. This is to give us hope that while it is true we are in exile, Hashem will always deal mercifully with us and will always give us a chance to repent, to improve, and to bring back the Beis Hamikdash. We should realize the opportunity that we have – that Hashem will respond to us if we are able to improve our actions, and will allow us to merit the rebuilding of the Bais Hamikdash.