Allah strikes the following parable in surah al-An’aam:
فَمَن يُرِدِ اللَّهُ أَن يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلْإِسْلَامِ ۖ وَمَن يُرِدْ أَن يُضِلَّهُ يَجْعَلْ صَدْرَهُ ضَيِّقًا حَرَجًا كَأَنَّمَا يَصَّعَّدُ فِي السَّمَاءِ ۚ كَذَٰلِكَ يَجْعَلُ اللَّهُ الرِّجْسَ عَلَى الَّذِينَ لَا يُؤْمِنُونَ
So whoever Allah wants to guide, He opens his chest to Islam. And whoever He wants to misguide, He makes his chest tight and constricted as though he were climbing up the sky. Thus does Allah place wrath upon those who do not believe. [6:125]
In his commentary on this parable, sheikh ‘Abd al-Rahman al-Sa’di wrote the following beautiful words:
يقول تعالى -مبينا لعباده علامة سعادة العبد وهدايته، وعلامة شقاوته وضلاله-: إن من انشرح صدره للإسلام، أي: اتسع وانفسح، فاستنار بنور الإيمان، وحيي بضوء اليقين، فاطمأنت بذلك نفسه، وأحب الخير، وطوعت له نفسه فعله، متلذذا به غير مستثقل، فإن هذا علامة على أن الله قد هداه، ومن عليه بالتوفيق، وسلوك أقوم الطريق. ـ
Here, Allah is making it clear to His servants what are the signs of a slave’s ultimate joy and guidance and what are the signs of a slave’s ultimate misery and misguidance. So He says that whoever’s chest has been opened up to Islam – that is, it has been made wide and spacious – then it becomes enlightened with the light of faith and enlivened with the light of certainty. So his heart becomes at peace with that, loves good, and becomes compliant in doing it – taking enjoyment from it, nor at all feeling burdened. So this is a sign that Allah has guided such a person and blessed him to be rightly guided and traversing the most upright path.