Word by Word Analysis

فَمَن يُرِدِ ٱللَّهُ أَن يَهْدِيَهُۥ يَشْرَحْ صَدْرَهُۥ لِلْإِسْلَـٰمِ ۖ وَمَن يُرِدْ أَن يُضِلَّهُۥ يَجْعَلْ صَدْرَهُۥ ضَيِّقًا حَرَجًا كَأَنَّمَا يَصَّعَّدُ فِى ٱلسَّمَآءِ ۚ كَذَٰلِكَ يَجْعَلُ ٱللَّهُ ٱلرِّجْسَ عَلَى ٱلَّذِينَ لَا يُؤْمِنُونَ 125

So whoever Allāh wants to guide - He expands his breast to [contain] Islām; and whoever He wants to send astray - 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.