Musar for Sanhedrin 197:5

And Rabbeinu Yonah has written in Iggereth Hatshuvah that "when men finish their work and their dealings and go to their houses or idle on the [street] corners or speak idle talk, their evil is very great and their sin exceedingly severe, for they [thereby] shame the Torah. For if they believed that there is no end to its reward, why would they not turn their feet to the house of study to learn? Does not a man rush to work knowing that all is vanity? And how can he forget the life of the world to come and not devote a day or an hour to learning? And let him not say: 'There is yet time to do for the L-rd, to learn Torah, and to occupy oneself with mitzvoth and tzedakoth.' He is guilty! He has sinned greatly against the L-rd! And Chazal have said on the verse (Bamidbar 15:31): 'For the word of the L-rd he has despised, and His commandments he has broken. Cut off shall be that soul; its transgression is in it,' that if it is possible for one to occupy himself with Torah but he does not do so, he shames the word of the L-rd. Therefore, every man is obligated to set aside a place in his house in which to learn halachoth or Scripture, each according to his ability. And when he is finished with his dealings or with his work, he must turn in there to study. And by doing so, he will "do wonders" for his soul, to rescue it from the pit. And he must reflect upon his end and consider his latter end, as Chazal have said: 'Consider three things and you will not come to transgression, etc.' And it is fitting that one find himself a pashut or a half [(types of coins)] to give charity for every day he fails to go to the house of study or to a place where he has one of the holy books to learn from."

Therefore, if one wishes to cleanse his soul, let him reflect upon the greatness of the issur of levity in several respects. For aside from the fact that he [the mocker] sins himself, it [levity] being one of the four things because of which one does not receive the Shechinah, as Chazal have said (Sotah 42a), he also causes many to sin, bringing them to multiply mockery. And the punishment of causing many to sin is well known, it not being granted one to repent because of this, as Chazal have stated (Avoth 5:18). And, aside from all this, involvement in idle talk, even if it contained nothing forbidden, causes one to lose the time that he could have spent in Torah study and the acquisition of the world to come. And he gives the impression that he scorns the words of the L-rd and the reward of the world to come, as stated in Sanhedrin 99a on the verse (Bamidbar 15:31): "For he has scorned the word of the L-rd" — R. Nehorai said: ['This refers to] one who could have studied Torah but did not do so.'"

It is public knowledge to all of Israel, both young and old alike, that the Torah is read, with the sanctity of a sefer Torah, only when it is entirely written without even one letter missing. Only then is it called (Psalms 19:8): "The Torah of the L-rd is complete." But if a letter is lacking, it does not possess the sanctity of a sefer Torah, but only the sanctity of parshiyoth. So is it with the Jew. When is he included in the congregation of Israel, having a share in the world to come? Only when he believes that the entire Torah was given by the L-rd from heaven. But one who denies it — even one letter of it — is not in the congregation of Israel (see Sanhedrin 99a; Rambam, Hilchoth Teshuvah 3:5; Yoreh Deah 158).