The haftorah of parshas Ki Sisa describes the showdown between Eliyahu Hanavi and the prophets of the Baal. The event begins with Eliyahu's rebuke to Bnei Yisrael, "Until when will you dance between the two opinions? If Hashem is the G-d follow Him and if the Baal [is god] follow him" (Melachim I 18:21). From this declaration, Rav Wolbe (Da'as Shlomo) deduces that it is preferable to solely worship an idol than to append idol worship to one's avodas Hashem!
Rabbeinu Yonah (Avos 1:5) writes that, "Thoughts of Torah and thoughts of promiscuity are two opposing thoughts which the heart cannot tolerate together." The same applies to Torah observance and idolatry: They are two incompatible identities that cannot live in consonance with each other. Torah requires temimus - absolute purity, and Torah influenced by idolatry is not true Torah. Thus one is better off solely worshipping an idol than adding idol worship to his avodas Hashem thereby corrupting the Torah.
Chazal chose this event as the haftorah for Ki Sisa because there is a correlation between the story of Eliyahu and the sin of the golden calf. When Bnei Yisrael committed the sin of the golden calf, Hashem told Moshe, "I have seen this nation and behold they are a stiff-necked nation. Now desist from me...and I will annihilate them" (Shemos 32:9-10). The impetus for the annihilation was not the sin of idolatry; it was the negative middah of being stiff-necked which caused the sin!
What exactly is this stubbornness referred to by the Torah as being stiff-necked (keshei oref)? It is the lack of ability to give up the routine or habitual practices that one grew up with, despite the fact that he realizes that in his present state they are no longer appropriate. When Moshe failed to appear, they were afraid to face a future without him and therefore they looked for a replacement. The inclination to idolatry, which had become a matter of habit in Mitzrayim, resurfaced and galvanized them to create the golden calf. (Although the eirev rav were responsible for the actual creation of the golden calf, Bnei Yisrael failed to rebuke them and therefore were also held accountable.)
While the fear of a future without Moshe, and the stubbornness which caused them to cling to their idolatrous tendencies of the past seem to be two totally unrelated aspects of the aveirah, nevertheless, there is a common denominator between them: both lack the temimus that the Torah necessitates. The Torah commands us, "You shall be tamim (wholehearted) with Hashem" (Devarim 18:13). Rashi explains that one should not run after fortune-tellers and astrologers in order to find out what the future will bring; instead he should rely on Hashem and accept all that He dispenses. Temimus is the ability to live wholeheartedly with Hashem; not to stubbornly hold onto harmful predispositions and not to worry about the future. True Torah observance requires absolute purity without antithetical ideas sullying its waters.
When Eliyahu Hanavi demanded temimus from Bnei Yisrael, he was demanding avodas Hashem without idolatry. While we no longer have the temptation of idolatry, many still "dance between two opinions." We might learn and daven, perform chessed and fulfill mitzvos, and nevertheless bring into our homes items or hashkafos that are completely antithetical to Torah true Judaism! Confront yourself and see if you are not indeed straddling the fence when it comes to your avodas Hashem. This exercise is not merely extra credit; it is an absolute necessity for one who wishes to truly serve Hashem.