Paul had been a devoutly religious Jewish theologian.  Before he met Jesus, he believed that his religious observance would make him right with God.  In other words, Paul relied on his own ability to keep the Jewish law to gain God’s approval.  His standing before God depended on what he could do (Philippians 3:4-6).  

But after meeting Jesus, Paul realised that he had been deluded.  He realised that trying to be good could never make him perfect in God’s sight.  The law can never make somebody righteous; it can only point out their sin.  Those who rely on getting everything right put themselves under a curse when they fail in even the smallest thing (Deuteronomy 27:26).  
 
Faith is so different to law.  The law given to Moses says that all who sin will die (Ezekiel 18:4); but faith trusts God’s promise that those who trust in Jesus will have their sins taken away (John 3:16).  The difference is massive.  And yet many in Paul’s day were deluded by false teachers into thinking that if they did good things, God would accept them.  The reality was that unless they received Jesus, their sins would curse them for ever.
 
Nothing has changed.  It is the same today.  It seems a paradox that those who try hard to please God are not accepted if they have not asked Jesus to take their sins away (John 3:18).  It seems strange to many that all those who are bad will be accepted if they repent and trust in the atoning sacrifice of Christ. That is why the gospel is such good news for bad, failing, sinful people; and such an embarrassment to nice, kind and outwardly good people who do not think they need saving.  Jesus said, “It is not the healthy who need a doctor, but those who are ill. I have not come to call the righteous, but sinners to repentance.” (Luke 5:31-32).  Jesus is still calling sinners; it is our job to pass the message on wherever people will listen.