Look

"If you’re a Calvinist you might ask, am I elect? If you’re not a Calvinist you just might ask, is my faith authentic? It’s the same kind of problem experienced at the same level. And the bottomline answer to that is not a simple little “here it says, ‘If you believe, you have the Holy Spirit.’” Have you believed? Yes. Where’s the Holy Spirit? He’s in my heart. That does not work. That simply does not work. That is so superficial, because the issue is, am I really believing? Because the Bible says there are going to be some people in the last day who are stunned when he says, “I never knew you; depart from me” (Matt. 7:23). They’re going to think they were believing all the time, but they were not believing. So how do I know if I am believing? That’s the kind of terror that will keep you awake at night and make life really hard. The bottom-line answer is: Look to Christ. Look to Christ. Look to Christ. Only in looking to Christ and the cross does Romans 8:16 powerfully happen. “The Spirit bears witness with our spirit that we are the children of God.” I can’t give anybody assurance that they’re truly saved.

I can’t give anybody assurance that they’re elect. But God can. And it’s a miracle. You pray for it and you wait for it, and you don’t stand in front of the mirror looking endlessly into your soul with introspection. That comes periodically, but mainly you stand in front of the cross and you keep looking and looking and looking. And in looking you are saved." (emphases mine)

- John Piper, Suffering and the Sovereignty of God, Appendix: An Interview with John Piper, pp. 229