This verse introduces a logical inference, so then, drawn on the basis of the scripture quotation in the previous verse. The Good News Translation rendering is somewhat different from a literal rendering of the Greek text, “So then, not of the one who wills or of the one who runs, but of God who shows mercy.” The Revised Standard Version translates: “So it depends not upon man’s will or exertion, but upon God’s mercy;” and the New English Bible: “Thus it does not depend on man’s will or effort, but on God’s mercy.” All three translations take the verb “to run” in the more general sense of “to exert oneself” (Good News Translation does; Revised Standard Version “exertion”; New English Bible “effort”). Although the pronoun it does not appear in the Greek text as such, a good English translation of Paul’s words almost requires this type of construction. In fact, many English translations use a construction very similar to that employed by the Good News Translation (Jerusalem Bible is an exception: “the only thing that counts is not what human beings want or try to do…”).
In a number of languages one must specify what the pronoun it refers to. In the present passage, it has reference to God’s choice. Therefore, one may render verse 16 as “hence, whether God chooses a person or not does not depend on what a man wants or does, but only upon the mercy which God shows” or “… is not because of what a man wants or does but only because God wants to show mercy.” In this type of context “to show mercy” must be understood in a very broad sense of “do good to” or “to cause good for.”
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 .