Potential converts are often told that accepting Jesus is their only chance to avoid burning in hell for all of eternity. Belief in Jesus is saviour is a 'get out of hell free' card. Missionaries love to claim that Jews have 'nothing to lose and everything to gain' by accepting Jesus as their saviour.
To this, it is necessary to respond that if Jesus is not a deity, worshipping him as G-d constitutes idolatry. The punishment of an idolater is spiritual death, and being cut off forever from G-d in the World to Come, karet. "But the soul that sinneth high-handedly, whether he be home-born or a stranger, the same blasphemeth the LORD; and that soul shall be cut off from among his people. Because he hath despised the word of the LORD, and hath broken His commandment; that soul shall utterly be cut off, his iniquity shall be upon him." (Num. 15:30-31). Idolatry is one of the the three cardinal sins which a Jew must give his life before committing. Indeed, throughout the ages, Jews have prefered death than baptism. During the Crusades, tens of communities were destroyed and thousands of Jews were massacred, yet only a few individuals accepted Christianity. Most sang the Shemah, the Jewish testament to G-d's Unity, before being slaughtered.
G-d lay our eternal allegiance after rescuing us from the Egyptian bondage. At Sinai, He told us that we should have no other gods before Him ie. as intercessors between Him and us. There is no need for an intermediary to reach HaShem; He saved us Himself and wants a personal and direct relationship with all of us. "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). This applies equally to Jews who accept false saviour-gods, thinking that this will lead them closer to G-d. G-d explained at Sinai exactly how He should be worshipped. Any deviance from what G-d commanded us at Sinai is idolatry. G-d told us specifically that "G-d is not a man, that He should lie" (Numbers 23:19) This same concept is repeated in I Samuel 15:29. G-d has no form or image and in non-corporeal. He is unbound by time and space, completely limitless and therefore cannot be restricted by physical shapes. This was the sin of the Jews who worshipped the Golden Calf. They believed that Moses had died and they desired a god to serve as an intermediary between them and HaShem. "Take ye therefore good heed unto yourselves--for ye saw no manner of form on the day that the LORD spoke unto you in Horeb out of the midst of the fire-- lest ye deal corruptly, and make you a graven image, even the form of any figure, the likeness of male or female." (Deut. 4:15-16) Any attempt to give G-d a physical image places limits on Him, the Unbound and Limitless One.
A Jew is meant to bear witness to the Unity of G-d. "Ye are My witnesses, saith the LORD, and My servant whom I have chosen; that ye may know and believe Me, and understand that I am He; before Me there was no god formed, neither shall any be after Me." (Isaiah 43:10) Twice a day, in the morning and evening, and then again before he goes to sleep, a Jew recites the Shemah and testifies that HaShem is One. These words are contained in the tefillin which he dons every weekday, and engraved on the mezuzot on his doorpost. These are the very last words that a Jew should say before he leaves this world. When a Jew accepts Christianity, he loses his exalted status and commits idolatry, invoking the sin of karet. Far from gaining G-d's grace, by believing in Jesus as a divinity, a Jew loses all of his share in the World to Come and is eternally cut off from G-d.
"Return, O Israel, unto the LORD thy God; for thou hast stumbled in thine iniquity." (Hosea 14:1)