Our Gemara on Amud Beis tells us:

שִׁדְרוֹ שֶׁל אָדָם – לְאַחַר שֶׁבַע שָׁנִים נַעֲשֶׂה נָחָשׁ. וְהָנֵי מִילֵּי דְּלָא כָּרַע בְּ״מוֹדִים״.

A person’s spine, seven years after his death, metamorphoses into a snake. The Gemara qualifies the last statement: And this matter applies only to a case where that person did not bow during the blessing of thanksgiving, the eighteenth blessing of the Amida prayer.

Tosafos brings two peshatim.  

1.  What relationship is there between the sin of not bowing by modim and the spine becoming a snake? Because it is a mitzvah to bow when reciting modim, and when straightening after bowing, one must straighten himself like a snake, who raises his head first and then his body, so that it should not appear as if the bowing is a burden upon him, as we find about Rav Shaishes in Berachos (14b).The punishment is measure for measure, since he did not bow and then raise his spine like a snake does, his spine becomes a snake. This is his punishment, for it is humiliating for him that his spine becomes a snake
2. Because the Midrash says that there is a bone (vertebrae) in the spine of a person from which he is resurrected in the time to come, when Hashem will resurrect the dead, and that bone is so strong and hard that fire cannot consume it. Thus, it survives and will be the initial component from which his body will be resurrected. And now, when that bone becomes a snake, he will not be resurrected because the bone that is used as the initial component of his resurrected body will not be available because it is a snake and will not live in the time to come, when the dead will be resurrected.

Tosafos rejects the second peshat as too harsh a punishment and therefore illogical. Shall we really accept that a person would lose his share in the world to come just because he did not bow in Modim?  Tosafos asserts that all Jews have a share in the world to come (Sanhedrin 90a). 

How can we understand the position of those who favored the second peshat?  Rav Tzaddok (Tzidkat HaTzaddik 147) understands this as a metaphor. It is not literally one who does not bow in Modim, but rather one who does not humble himself and show fear before God.  And in actuality, Rav Tzaddok says, no Jew is that arrogant (as they are naturally humble see Yevamos 79a) and thus indeed all Jews do have a share in the world to come.

Similarly, Malbim (Bamidbar 21:7) also understands the refusal to bow as a metaphor, but a different shortcoming. He says it is the refusal to bow in penitence. Thus, one who refuses to repent is liable to lose his share in the world to come.  Whether it is arrogance or lack of repentance, the stakes are high, so we must work to overcome these human foibles.