Why did martin luther king and his followers oppose the granting of indulgences

Committed to the idea that salvation could be reached through faith and by divine grace only, Luther vigorously objected to the corrupt practice of selling indulgences.

Explanation:

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.