So whoever Allāh wants to guide - He expands his breast to [contain] Islām; and whoever He wants to send astray1 - He makes his breast tight and constricted as though he were climbing into the sky. Thus does Allāh place defilement upon those who do not believe.
Translation: Saheeh International (English)
1 As a result of the person's arrogance and persistence in sin.
Whomever Allah wishes to guide to the right path, He opens their heart to Islam so they accept it willingly. Whomever He wishes to abandon and not guide, He makes their chest extremely narrow and unable to accept the truth, such that they become like a person climbing into the higher layers of the atmosphere while finding it increasingly difficult to breathe. Just as Allah made this extreme narrowness the condition of the person who goes astray, He will place the punishment on those who do not have faith in Him.