Believing "just in case"
Answer: Actually, belief in Jesus is a horrendous sin with grave consequences for any Jew who professes to do so. To understand this let us look at the tragic apostasy of the Northern Kingdom of Israel and its dominant tribe, Ephraim. God, speaking through the prophet Hosea declares, "When Ephraim spoke with trembling, he became exalted in Israel; but when he became guilty through Baal, he died" (Hosea 13:1). When they served God, Ephraim was "exalted," but when they became guilty of idol worship and remained unrepentant despite prophetic warnings to cease their sinful ways they signed their own death warrant. Ephraim died a spiritual death long before it suffered national destruction.
From the prophetic message we learn that apostates, even during their lifetime are reckoned as dead as long as they remain unrepentant. The prophet not only denounces belief in molten images fashioned by craftsmen, but also those who trust in a false savior- god: "And I am the Lord your God from the land of Egypt, and gods beside Me you should not know, and there is no savior but Me" (Hosea 13:4).
God desires the apostate's repentance and beckons him/her to renounce iniquity: "From the clutches of the grave I would ransom them, from death I would redeem them, I will be your words of death; I will decree the grave upon you. Remorse shall be hidden from My eyes" (Hosea 13:14).
For the apostate who does not repent, God says, "I will decree the grave upon you." In a more literal sense, "I will be the cause of your being cut off to the grave." Katavcha, from the verb ketev, denotes "cutting" (e.g. Psalms 91:6). Its primary meaning is "to cut," but in Hosea 13:14, ketev takes on the secondary meaning "decree." In Hebrew, the primary word for decree is gezayra, the root of which is gezer, "to cut." There are a number of word roots in Hebrew whose primary meaning is "cutting," yet have a secondary meaning of a final, permanent decision or ruling (e.g., pasak, chakak, gezer, charatz, karat).
As we see, the belief in the false savior-god Jesus is a grave sin. The unrepentant apostate is not only shunned and considered dead by the Jewish community. God Himself considers the unrepentant apostate as spiritually dead in this life and in physical death the apostate is all the more so "cut off."