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?
 

God's plan of salvation was shrouded in mystery until Jesus came (Ephesians 3:8-12).  It was a half-understood secret which the Israelites thought was exclusive for them alone.  God, was their God; and they thought that He would only speak to them; even though He had already told Abraham that 'all peoples on earth will be blessed through you' (Genesis 12:3).  But part of the 'mystery of salvation', was that salvation through the death of Christ would be for all, Jews and Gentiles alike (Ephesians 3:6).
 

Paul was writing from a Roman prison, shackled to chains (Colossians 4:3).  He is writing to congregations he did not personally know (Colossae, Hierapolis and Laodicea – Colossians 4:13) but he knew of their faith from Epaphras (Colossians 4:12) who was a fellow prisoner for the sake of Jesus (Philemon 1:23).  Their work was to pray for the churches, which they did in great detail and with much energy (Colossians 1:3-13; Colossians 4:12).  

Many lies are attractive, especially religious lies.  Indeed, every idea about God which does not accept the Son of God as Creator, Sustainer, Ruler, Saviour and Judge - is a deception (2 John 1:7).  Nevertheless, history is littered with theistic inventions which exclude, marginalise or alter the character of Christ.  That was the background to the Apostle Paul's ministry; and is the context in which the church proclaims Christ today.