What is the explanation of this statement? Does God not punish for violating His Law? Did not most of those who heard Moshe's words have relatives who died as a result of their sins at the time of the Meraglim or on other occasions?
I think that the message is that Hashem's covenant with His nation is two-fold. The covenant with individuals is rescindable, for if individuals desecrate the Torah with severe malice, God metes out punishment which results in termination of His relationship with the sinners (at least in this world). This occurred at the time of the Chet ha-Egel and the Meraglim. There is, however, another level of Hashem's covenant; His covenant also applies independently to the tzibbur (community) as a whole, and this covenant can never be rescinded. This is precisely Moshe Rabbeinu's message of comfort and pacification to the Jews.
How does this concept fit into the general theme of the parshah, which is teshuvah (repentance)? The answer is clear, as the unbreakable commitment of Hashem to His people (through His covenant with the tzibbur) signals that He is always available and in contact - we need only reach out, and He is there. This availability and approachability is the key to teshuvah, for it invites the person to address Hashem and call out to Him.
May we all reach out to Avinu - our Father - and renew our permanent covenant with Him.