Salaams... it is part of our Imaan to believe in Allah (swt's) mercy since Ar-Rahman is a part of Allah (swt's) Sifat as is often repeated in "Bismillah hir Rahman ir Rahim"
Also this ayat like many make Allah (swt's) mercy abundantly clear: "Say: O My servants who have transgressed against their own souls, despair not of the mercy of Allah. Indeed, Allah forgives all sins. Truly, He is Most Forgiving, Most Merciful." (Surah az-Zumar 39:53)