Adam Clarke on Colossians 2:7

Rooted and built up in him - It is not usual with the apostle to employ this double metaphor, taken partly from the growth of a tree and the increase of a building. They are to be rooted; as the good seed had been already sown, it is to take root, and the roots are to spread far, wide, and deep. They are to be grounded; as the foundation has already been laid, they are to build thereon. In the one case, they are to bear much fruit; in the other, they are to grow up to be a habitation of God through the Spirit. See the notes on Eph 2:21, Eph 2:22 (note); Eph 3:17 (note).

Abounding therein with thanksgiving - No limitation is ever set to the operations of God on the soul, or to the growth of the soul in the knowledge, love, and image of God. Those who are brought into such a state of salvation should abound in gratitude and loving obedience, as they grow in grace.