This verse has often been misunderstood.  Some think it means that Christ's work on the cross was insufficient to achieve salvation.  Not so.  The sacrificial work of the Lord Jesus Christ was complete, perfect and sufficient to pay for all the sin of the world (Hebrews 10:12-14).  So, what does this verse mean?
 
Clearly, the Apostle Paul had suffered much in proclaiming the gospel (2 Corinthians 11:23-28), but none of his sufferings could add anything to the finished work of Jesus Christ.  So, what did Paul mean by saying there was something 'lacking' in what Christ did?  As the redemptive purpose of the cross was complete in itself, the only thing that was lacking was people knowing about it.  That is why Paul suffered so much.  His suffering came because he proclaimed Christ crucified, preached the gospel and urged people to repent and submit to Jesus.  If he had not done that, people would never have known, they could not be saved and the church could not grow (Romans 10:13-15).
 
Sharing the gospel continues to be a hazardous task, simply because those who are in rebellion against Christ hate to be reminded of the truth that they are fighting against God.  Their easiest solution is to try to get rid of the messengers of the gospel.  That is why Paul was repeatedly attacked and other apostles also lost their lives.  But Peter explained Jesus’ command to keep on sharing the message of salvation in Acts 10:42-43, “He commanded us to preach to the people and to testify that he is the one whom God appointed as judge of the living and the dead. All the prophets testify about him that everyone who believes in him receives forgiveness of sins through his name.”

Today, the gospel is still such a wonderful message, and salvation is such a wonderful deliverance.  God’s people should still want to tell others so that they may also be saved and be a part of the eternal church of Jesus Christ.  So, if you encounter opposition, hostility, or other persecution for the sake of the gospel you should be encouraged (2 Timothy 3:12).  It means that the gospel message is starting to be heard and that people are reacting.  Although it may seem that everybody is against you, some people are actively and eagerly listening to find out how they may be saved.  Then all the suffering will be worthwhile for you and for Christ.