The quotation in this verse is from Exodus 33.19.

… on whom I wish … on whom I wish may be translated in some languages more effectively as conditional clauses—for example, “if I wish to have mercy on someone, I will; if wish to take pity on someone, I will do so.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .