Chasidut על שבת 210:8

However our teacher, Moses, did not only say, "And you shall know [...] that the Lord, He is God, in the heavens." He also said, "on the earth below; there is none else." And that is central - to see and know that also below on the earth, there is none besides God and His holiness, may He be blessed. But how can a person say that if, God forbid, there is a foreign god in his heart? For the Gemara says (Shabbat 105b), "What is the strange god that is within a person’s body? Say that it is the evil inclination." And how can a person say, "I am a Jew and I serve God," if he also worships, mutatis mutandis, the strange god inside him, listens to it and does its will, God forbid. How can he approach God, and say, "Blessed are You Lord, our God"; be in front of God and say, "Lord, You are our God" - when he has, mutatis mutandis, a strange god nesting in his heart to which he, God forbid, subjugates himself.

If we delve into the spirit of the youth and evaluate them with a pure and completely honest investigation, we would see that a lad will usually not veer from the ways of God all at once. He does not profane Shabbat instantly, nor does he suddenly go from the study hall to, mutatis mutandis, the theatres. The Gemara (Shabbat 105b) says, "That is the craft of the evil inclination. Today it tells him [to] do this, and tomorrow it tells him [to] do that, etc." At first glance, we see in the Gemara that only when he is already about to do bad things, God forbid, does he not begin with weighty sins from the start, but rather descends into the pit of destruction gradually. But as we see - even before he comes to the first, most insignificant deed - the internal break in his mind also does not happen suddenly and all at once. Rather a year ago or more, his mind became slightly infirm - infirm, but not infirm. 'It appears like a blemish in the house' - only like a blemish. And the smallest thought bubbles through from it, to look at the Torah and divine service as something disgraceful and as a waste of time; and at the outside world of licentiousness, ignorance and wildness as beautiful and good. Even the most insignificant thing can cause such an infirmity. Since at first, it is tiny and does not do much to him or his deeds, his rabbis - and even his closest friends - do not see any difference in him. But, little by little, it deepens itself into his spirit and spreads into his thoughts, to destroy him, from the spirit to the flesh. Then suddenly a loud voice is heard, that he has left the House of God and fallen into the pit of destruction, God forbid.