﷽
ما بعد
To begin:

عَنْ أَبِي رُقَيَّةَ تَمِيمِ بْنِ أَوْسٍ الدَّارِيِّ رَضِيَ اللهُ عَنْهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ: “الدِّينُ النَّصِيحَةُ.” قُلْنَا: لِمَنْ؟ قَالَ: “لِلَّهِ، وَلِكِتَابِهِ، وَلِرَسُولِهِ، وَلِأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ.”
The Prophet (ﷺ) said, “The deen (religion) is naseehah (advice, sincerity).” We said, “To whom?” He (ﷺ) said, “To Allah, His Book, His Messenger, and to the leaders of the Muslims and their common folk.”
[Muslim] [40 Hadith Nawawi]
This famous hadith may beg the question: grammatically, how does the word “al-nasiha” (النصيحة) change by making it definite (معرفة)?
When the mubtada’ and khabar (subject and predicate) are both definite, it indicates that the mubtada’ is entirely contained in the khabar. This may also be figurative, and not literal.
Possible translations: “Religion is sincerity.” Or “religion is but sincerity” or “religion is but good faith.”
Such as when I advise you in good faith or serve a leader in good faith, etc.
والله أعلم