Wise parents teach their children to say, and mean, three important words: 'please', 'sorry' and 'thank you'.   Yet all of them are against our human nature.  We would rather demand our rights than wait for somebody to be pleased to give; we resist admitting our failure; and feel that gratitude makes us indebted to others.  In the previous verse, Paul the Apostle encouraged the Colossian church to live maturely, "... and giving joyful thanks to the Father, who has qualified you to share in the inheritance of his holy people in the kingdom of light" (Colossians 1:12).
 
Paul gives two major reasons for which Christians should be thankful.  Firstly, God has rescued us from Satan's captivity (Hebrews 2:14-15), paying for our redemption with the blood of Christ (1 Peter 1:18-19).  Secondly, we have been fully welcomed into Christ's kingdom, because our sin (which barred us from coming close to Him) has been forgiven through the sacrifice of Christ.

Are we grateful (Colossians 1:12)?  If we are not, there is something seriously wrong!  Without God's grace, we can only live in spiritual darkness, fumbling our way through life with fear of hell and no hope of heaven. Gratitude says a lot about people.  It says that they are not proud, but willing to accept help; it says that they are not ashamed of being dependent; it says that they acknowledge they remain in the debt of others, which they can never repay except by saying 'thank you' (Ephesians 5:20).
 
Musicians practise on their instruments; lawyers practise their learned profession; carpenters practise their woodworking skills.  Christians practise their faith first by giving thanks to God, and then demonstrating their thankful hearts by behaving in a way which pleases the Lord (Colossians 1:10).  If lips refuse to sacrifice pride and give thanks - praise is always the sacrifice of self-interest (Hebrews 13:15) - the rest of the body is unlikely to behave with spiritual maturity.  If we do not remember the magnitude of our rescue, and give thanks to the Lord, we will tend to go back to old ways and be trapped in dark ways again.  If we do not remember what it cost Christ to forgive us, we will treat sin lightly.  And if we do not value the privilege of being in His kingdom we will inevitably undermine it.  The remedy is to give thanks with joy.  Start now and practise several times a day until it becomes the habit of a lifetime!