The Quran

فَأَقِم وَجهَكَ لِلدّينِ حَنيفًا ۚ فِطرَتَ اللَّهِ الَّتي فَطَرَ النّاسَ عَلَيها ۚ لا تَبديلَ لِخَلقِ اللَّهِ ۚ ذٰلِكَ الدّينُ القَيِّمُ وَلٰكِنَّ أَكثَرَ النّاسِ لا يَعلَمونَ

(Saheeh International)

So direct your face [i.e., self] toward the religion, inclining to truth. [Adhere to] the fiṭrah¹ of Allāh upon which He has created [all] people. No change should there be in the creation of Allāh.² That is the correct religion, but most of the people do not know.

The natural inborn inclination of man to worship his Creator prior to the corruption of his nature by external influences. Thus, Islāmic monotheism is described as the religion of fiṭrah –that of the inherent nature of mankind.
i.e., let people remain true to their fiṭrah within the religion of Islām.