Is a young man who has strayed from some of the teachings of his religion and been influenced by a foreign environment, while still maintaining his prayers, considered to be among those whose hearts have been sealed?

Engaging in and becoming addicted to sins leads to the blackening and darkening of the heart. The Prophet, peace and blessings be upon him, said: "Indeed, when a servant commits a sin, a black spot is marked on his heart. If he repents and seeks forgiveness, his heart is polished. But if he returns [to sin], it increases until it covers his heart. That is the 'Ran' (stain) which Allah the Almighty mentioned: 'No! Rather, what they used to earn has covered their hearts.'" Hasten to repent before death, for Allah rejoices at the repentance of His servant and accepts it, transforming his evil deeds into good ones: "Do they not know that Allah is He who accepts repentance from His servants?" and "Except for those who repent, believe, and do righteous deeds - for those, Allah will change their evil deeds into good ones." Beware of Satan's despairing you from the mercy of Allah.