When the spies return to the camp and disparage the land, the Jewish people believe their negative report. Consequently, Hashem tells Moshe that he will eradicate the Jews in a plague, and start a new nation from Moshe. However, Moshe is able to convince Hashem not to kill the Jews; he beseeches God to forgive the people, telling Hashem that the people of Mitzrayim will say that You, Hashem, are all powerful if you don’t kill the Jews. Hashem agrees not to kill the Jews: “VaYomer Hashem Salachti KiDvarecha” (BeMidbar 14:20). Yet Hashem seemingly reverses his previous statement only two Pesukim later, saying: “Ki Kol Ha’Anashim HaRo’im Et Kevodi Ve’Et Ototai Asher Asiti BeMitzrayim UBaMidbar VaYinasu Oti Zeh Eser Pa’amim VeLo Shamu BeKoli. Im Yiru Et Ha’Aretz Asher Nishbati La’Avotam VeKol Mina’atzai Lo Yira’uha” (14:22-23). So why does Hashem seemingly retract his earlier statement to forgive Am Yisrael?
The Chizkuni explains as follows: when Hashem tells Moshe that He has “Salachti KiDvarecha,” it is in the past tense. Hashem is saying that yes, I told you that if the Jews ever sin, then you can invoke my 13 attributes and they will be forgiven; however some sins are too large to be forgiven. I forgave the Eigel HaZahav, but you are still punished throughout the ages for it, slowly paying for the sin-- “VeTamid Ke’Efkod Aleihem Avonoteihem UPakadeti Aleihem Me’at Min Ha’Avon HaZeh Im She’ar Ha’Avonot” (Rashi Exodus 32:34). Hashem says that the punishment for this sin is too large to punish on just one generation, it must be extended. Yet, the Ramban says that the sin of the golden calf was not an attack on God, they were looking for a new leader to be a messenger of God; “Aval Hayu Mevakshin Moshe, Amru, Moshe SheHorah Lanu HaDerech MiMitzrayim Ve’Ad Hinei, SheHayu HaMa’asim Al Pi Hashem BeYad Moshe, Hinei Avad Mimenu. Na’aseh Lanu Moshe Acher SheYoreh HaDerech Lifneinu Al Pi Hashem Beyado. VeZeh Ta’am Hizkiram Moshe Ha’Ish Asher He’elanu, Lo HaKeil Asher Ha’Olam, Ki Yetztarchu Le’Ish Elokim” (Ramban Exodus 32:1). A sin that is described as a rebellion against God, however, would entail an even larger punishment. Therefore he had extended the sin of the spies forever, it will be too large; too great of a punishment for us to ever overcome.
However, the Chizkuni also offers another interpretation of this Pasuk; when Hashem says "Salachti KiDvarecha”, Hashem is saying that you, Moshe, evoked the thirteen attributes of Hashem-- in the verse that you stated it is written that I hold the sins of the fathers onto their next generations. Therefore I will forgive them by not punishing the next three generations; rather, I will allow this generation to die slowly, and that will be the continued punishment for the next generation.
In contrast, the Seforno believes that Hashem was being merciful when He slowly killed the Jewish people. He had planned to kill us all in a plague, to eradicate the Jewish people swiftly and permanently; yet, Moshe convinced Hashem to kill the generation of sinners slowly. This allowed the next generation to grow older and take the place of those who die every year because of the punishment.
When the latter positions of the Chizkuni and the Sforno’s opinion are synthesized, it is clear what Hashem means when he says that “He has forgiven us”. Hashem is being merciful by killing the generation of the sinners slowly. He allows the Jewish people to continue to thrive even after a tragedy with the magnitude of the Eigel HaZahav. He shows that He cares about us and wants us to survive as a nation. He will always look upon us with favor and mercy; as the Ibn Ezra comments to the next Pasuk, while Hashem is still “living,” so too what He says will be true. Hashem will punish us in an honorable way; it will be in such a manner that even the other nations will see Hashem’s glory. The nations of the land can never say that God does not have the power to be merciful, and even though this generation will never see the land, Hashem will ultimately give us Israel (Bechor Shor BeMidbar 14:21).