It is from a slave’s understanding of the deen..

أَنَّ أَبَا الدَّرْدَاءِ ، قَالَ : ” إِنَّ مِنْ فِقْهِ الْعَبْدِ أَنْ يَتَعَاهَدَ إِيمَانَهُ وَمَا نَقَصَ مِنْهُ ، وَمِنْ فِقْهِ الْعَبْدِ أَنْ يَعْلَمَ أَمُزْدَادٌ هُوَ أَمْ مُنْتَقِصٌ ؟ وَإِنَّ مِنْ فِقْهِ الرَّجُلِ أَنْ يَعْلَمَ نَزَغَاتِ الشَّيْطَانِ أَنَّى تَأْتِيهِ ؟

شرح أصول اعتقاد أهل السنة والجماعة اللالكائي

Abu darda’ رضي الله عنه said:

It is from the slave’s understanding (of religion) to take care of his emaan and beware of what decreases it, andto understand if his emaan is increasing or decreasing. And to know the ways shaytan tries to tempt him.