In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Imaam As-Sadi [may Allaah have mercy upon him] said:

It (i.e. Islam) is the religion whose adherents – those who established it and embodied its guidance and teachings- gave the hearts access to good with knowledge and sound faith, and they were victorious over the inhabitants of the various regions of the earth through justice, mercy and sincere advice to the human race.

Commentary: The hearts were given access to good through the sciences of this religion and what it calls to through knowledge and sound faith- sound belief in Allaah and everything that Allaah has commanded His servants to believe in. They (i.e. the true adherents to the religion) – by way of their adherence to the religion in belief, worship and good manners – were the ones through whom the hearts were given access to good (by the will of Allaah). Ponder upon this affair based on the statement of Allaah:

[ فَبِمَا رَحۡمَةٍ۬ مِّنَ ٱللَّهِ لِنتَ لَهُمۡ‌ۖ وَلَوۡ كُنتَ فَظًّا غَلِيظَ ٱلۡقَلۡبِ لَٱنفَضُّواْ مِنۡ حَوۡلِكَ‌ۖ – And by the Mercy of Allah, you (O Muhammad) dealt with them gently. And had you been severe and harsh-hearted, they would have broken away from about you]. [Surah Aal Imran. Aayah 159]

They gave access to the hearts to reach good through knowledge and sound faith, and they were victorious over the inhabitants of the lands, cities and countries through justice, mercy and sincere advice to the human race (by the will of Allaah). (1)

[Ref 1: From 31mins 03secs to 32mins 34secs. https://www.al-badr.net/detail/8LD6UgwQrZ paraphrased except]