And I am sure of this, that He who began a good work in you will bring it to completion at the day of Jesus Christ.
God has begun a good work in us. He has started it, but it is not yet complete. This is not the idea of somebody becoming a Christian and then basically sion then killing time until they go to heaven. This is an ongoing process, being made into his likeness, being made more like Jesus. This is described as a GOOD work, although often it wont seem like it to us! God often uses suffering to turn our hearts towards Him. But while we may not see that as good, God knows more about what is good for us than we do. We may think that a comfortable life is good for us, but God knows that situations that cause us to trust in Him are better. God is good, and He is more committed to our good than we are.