In these verses Paul uses two illustrations to help explain how Christians should live. The first picture, rooted, comes from horticulture. We know that a plant must have deep roots in good soil in order for it to grow. What we see above the ground depends on what is happening below the ground. When a sinner becomes a Christian, he is united to Christ. His faith is rooted in Christ – the participle is passive, indicating that God was the one who did the rooting.
As we think about this imagery, we can see that this initial activity is concealed from the view of others. Just as the roots of a tree are hidden below the ground, so the roots of faith are hidden in Christ. Another person does not know when this union takes place between Jesus and a sinner.
Further, we can see that what is important is the contact that the roots of faith have with the perfect soil (Christ). The faith of every believing sinner is weak and imperfect, but what gives hope is the state of the soil. There is in Jesus everything that each converted person needs in order to grow.
A third detail that this rooting brings to mind is the continuity expressed. The rooting in Christ will not end in disaster. Instead the sinner who trusts in Jesus can have confidence that the ingredients in the soil (in Christ) will ensure that the relationship is permanent.
The second picture that Paul uses is taken from construction. He is mixing his metaphors because he regards the rooting as the foundation on which the building is erected. The up-building in Christ is the equivalent of walking in Christ (just as the rooting in Christ is the equivalent of receiving him as Lord).
The point about a building is that it is visible. No one can see the foundation, but all can see the structure that is erected on it. Similarly no one can see the roots that a person has in Christ, but all will see the development of it as the individual lives for Jesus.
Not only is the structure visible, it is also incomplete. Paul does not have in mind a finished building, but one that is in process. This means that the Colossians will be in different stages of development. In a spiritual sense, they will not all be the same size. Also none of them have reached the final stage yet. Until that happens, they have to keep on walking in Christ.
A third aspect of the building is that it is gracious because the various features of the building arise out of God’s grace. Every item in the building is an expression of the work of the Builder. Jesus is making each of the Colossians into something beautiful and attractive.
The term means that when a believer is built up, his understanding of the doctrines of the faith is strengthened by God. As long as he continues to walk in Christ, the believer will continually appreciate the truths of the faith. The implication is that a failure to continue walking in Christ will result in spiritual weakness. As physical exercise establishes bodily strength, so spiritual exercise is the path for spiritual assurance.