Not so much a Sin, as...
During 13-month years (37% of all years), Parshat Ki Tisa is read before we get involved with the special Four Parshiyot. Another 21% of the time, during 12-month years that begin on Shabbat, Ki Tisa is a break-Shabbat among the Four Parshiyot. That means that 58% of the time, Ki Tisa is a one-Torah Shabbat. The remaining 42% of the time, Ki Tisa is Shabbat Para, a two-Torah Shabbat. This has nothing directly to do with the topic of the Lead Tidbit, but is included here to satisfy the curiosity of those who enjoy statistics.

To be sure, the Golden Calf was the serious subject of sinning. That’s why it is referred to as CHEIT HA’EIGEL. And that is why 3000 men were struck down as a result of what they had done. But it is important for the rest of us, the overwhelming majority of Jews, to see the episode and its aftermath as something else as well. It helped define human nature. It shows us who we are, at least most of us.

Human beings, and even that special sub-set of humans known as Jews, have always had trouble relating to an invisible, intangible G-d. Even when we heard His voice, we were still stymied by not “seeing” Him. Apparently, as long as Moshe Rabeinu was perceived as “spokesperson” of G-d, (and ours before G-d), the people were able to cope. Once Moshe went “missing”, the people panicked. And the ultimate expression of that panic was to declare the Calf as EILEH ELOHECHA YISRAEL. (Rashi says that it was the Egyptian tag-alongs that started the trouble with the Calf, and then they negatively influenced some of Bnei Yisrael.) Be that as it may, it is a terrible thing to say, and a terrible thing to hear. And when CHUR, son of Miriam, tried to stop the people from reveling in the Calf, he was killed for his troubles. And that made what was happening all the worse.

Yet except for the 3000 (less than ½% of the adult male population), the rest of the people were essentially forgiven by G-d. And more than that, although G-d distanced Himself from the people, He taught them the “secrets” of the 13 Divine Attributes. G-d “threatened” to destroy the people, but obviously He had no intention of doing so, because Moshe would not have been able to “disuade” Him if that were the case.

Evern when we speak of the Para Aduma as an atonement for the Sin of the Golden Calf, we are not talking about a korban (sacrifice) but rather about something that redefines the body component of the body-soul partnership.
And especially, the fact that the whole Mikdash concept was diverted, so to speak, to refocus the spiritual energy of the people, tells us a lot. G-d did not say, “That’s it! Forget the command to build a Mikdash. I don’t want it!” Rather the opposite was true. He taught us how to get closer to Him.