Romans 9:16 - Understood in Context

"So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy"

What is "it"?

That means, the availability of a personal experience of salvation to both Jews and Gentiles was not by man's will or works - but by God - it was only through God's own goodness that this was made available to all.

Paul's purpose in this passage appears to have been to complete a lack of understanding that the Roman believers had regarding why certain events had emerged (or, failed to emerge) the way they did in the first-century church and the highmindedness that was the result of some wrong assumptions along that line (Romans 11:20).

The emergence in the first century of a predominantly Gentile church rather than a Jewish church was the consequence of faith and unbelief and only in accordance with God Himself and His Word and His principles. God's promises hadn't failed - and neither was it a basis for Gentiles believers to hold their privilege with God lightly.

Israel was apparently failing to receive what the Prophets had promised; and meanwhile, Gentile believers were inheriting what was promised. Therefore some Roman believers may have been confused as to whether or not God's promises had failed; or some may have been beginning to wonder whether God had closed the door to salvation to Jews entirely; others may have been in danger of settling on their lees and becoming complacent about their own place, as Gentiles, in God's plans.

So it appears Paul sought to demonstrate the manner in which God's promises had in fact been fulfilled, and to caution against highmindedness. It was God Himself who had determined to make His mercy available to faith and to assert His severity against unbelief. The outcome that the Roman believers in the first century were witnessing all started with God and His principles - not with man's will or works - irrespective of whether they were Jew or Gentile. Therefore it behoved them to fear God.

It was by God's mercy that the Gentile Romans had been allowed to be included in the promises originally made to Israelis. And similarly, it was by God's decree that unbelieving Jews be cut-off from inheriting that which was potentially theirs.

It was God's own sovereignty that had determined that believers should be shown mercy and that unbelievers should be cut off - without respect to whether they were Jew or Gentile. Therefore believers should be in fear of God.

Paul was completing their understanding; and the the caution was against highmindedness (Romans 11:20) - therefore Paul's statements are to be understood within the context of that argument.