A person should not mistakenly think that he can be a kosher Jew without learning Torah. Our Sages compared a Jew who does not learn to a fish out of water. If one looks around, he can verify this, because when a person stops learning he spiritually flops around like a fish and finally his spirit dies within him. As long as he is still ‘wet’ with Torah he can survive, but when he ‘dries up’ he perishes spiritually. While some Torah is left, he will still do mitzvas, but when that dries up, his service of Hashem will become petrified and finally discarded. Spiritual life comes only from Torah, and without it there is no existence in the world of truth.
This is insinuated in the verse [as our Sages explained it, quoted in Rashi], ‘If you do not listen to me [to toil in Torah] and do not do these mitzvas [if you don’t learn, you will not do mitzvas], and if my mitzvas are repugnant to you [you despise people who do mitzvas], and my statutes are distasteful to you [you hate Torah sages], and do not do [causing others not to do mitzvas] all my mitzvas [you deny I commanded them] to deny my covenant [denying My existence] etc. You see from here that these seven levels of denial of Hashem all start with not learning Torah.