How do you know if you are a genuine believer? This is the most important question that you will ever need to answer. It is the same question that Jesus asked in Matthew 22:42 - "What think ye of Christ? Whose Son is He?"
There are certainly a number of biblical "tests" that can be applied to answer this all important question. One of the "best" tests is found in Romans 3:21 encompassed in the words, "But now..." Is there something that emanates from your soul that compels you to say, "Thank God!" Do you have a "but God" moment in your life?
Beginning in chapter 1, verse eighteen and running through chapter 3 and verse twenty, the Apostle Paul, under the inspiration of the Holy Spirit has been demonstrating the absolute hopelessness and sinfulness of every member of the human race. Get this straight, no one, no Jew or Gentile can ever become a Christian without coming to a realization that you are utterly hopeless, enslaved in and by sin, and unable to merit, earn, or otherwise obtain the righteousness required by God to be fit for His presence.
The Holy Spirit must work a work in your heart convincing you of the strategic fact that apart from Christ you are utterly hopeless and infinitely helpless. When you are brought to this point this is only fitting to see the "but now" of God.
Lloyd-Jones once wrote that these two words are the most wonderful words of Scripture. This is how Paul chose to introduce the gospel to the Romans. First hopelessness, then "but now..."
This is why I ask if there is a but now moment in your life. This is why it is such a great test of determining whether you are a genuine believer accepted by God according to the Scripture. If you have never been brought to utter helplessness before God then you have never had a "but now" experience. You have flunked the "test." You need to cry out to God, "O God what must I do to be saved?" The answer is ask God for the "but now" moment. But now the righteousness of God is revealed - cry out to God for salvation through Jesus Christ.