"Rooted" and "built up" give two comparisons. "Rooted" compares them with a tree. "Built up" compares them with a building. A tree sends its roots deep into the earth and stands firm, without being moved. By those same roots it draws nourishing sap from the earth.
As a tree is deepened into the earth by its roots, so you must remain deepened into the Lord Jesus by all the roots of your life. Draw from Him the sap of spiritual and grace-filled life.
Do not tear yourselves out of Him by these roots in order to be transplanted to something else. If you transplant a tree, it will take root elsewhere and live as it should. But if you are torn out of Christ the Lord and rooted in anything or anyone else, you cannot live as you should.
You will seem to live, but you will not truly be alive. For the one source of spiritual and grace-filled life is Christ the Lord. Whoever does not abide in the Lord can neither live nor bear fruit (cf. Jn 15:1-6).
Yet the chief thing the Apostle wishes to suggest is immovability in faith and convictions. Saint Chrysostom says: "'Rooted' means established. Do not be carried now to one path, now to another, but be rooted. What is rooted is never moved. Do you see what precise expressions Paul uses?"
"Do not act in such a way as to walk now in Christ, now in someone else" (Oecumenius). Therefore this means: "Hold firm faith in Christ the Lord" (blessed Theodoret).
"Built up," εποικοδομουμενοι, means being edified or constructed. This is another comparison, with a building, but not with one already completed. It is still being built.
Spiritual and grace-filled life in the Lord is not immobility. It is in constant movement, ever forward and ever higher. As soon as this movement stops, life itself also stops. Yet this movement is not relocation or change, but growth within itself.
This growth also takes place in Christ Jesus. Saint Paul could have said, "rooted and growing in Him." But another comparison, familiar to him, came to mind, the believer as a building. So he said, "being built up."
He does not disturb the thought, but rather enriches it. He sets two comparisons before the eyes of the mind and lets the readers supply what each lacks for the fullness of the thought: growth in the first, foundation in the second.
"Being built up" gives to the first comparison the thought of increase. "Rooted" gives to the second the thought of foundation. For what the root is to the plant, the foundation is to the building. What construction is to the building, growth is to the tree.
"Being built up in Him," that is, in the Lord Jesus. He is the foundation. He is also the builder, for without Him we can do nothing. He is also the plan of the building, for we must be conformed to His image (cf. Rom 8:29) and transformed into His image (cf. 2 Cor 3:18).
He is also the necessary bonds of the building. Only the material is supplied by the will from nature. Yet even this does not remain in its natural, raw state, but is remade by the grace of the Lord.
Thus one can truly be built, just as one can grow, only in the Lord. Whoever is in the Lord by faith and hope is already being built. As soon as one enters communion with the Lord, the Lord's constructive power begins to act in him.
Oecumenius writes: "εποικοδομουμενοι means growing upon the foundation of Christ." Theodoret says: "By the word 'built up' he expresses the acquisition of virtue." Saint Chrysostom says: "By inward disposition striving to reach His measure."
These two comparisons are interpreted, or applied to Christian life, in the following words. "Rooted" is interpreted by "established," and "built up" by "abounding."
"Established," βεβαιουμενοι, means standing unshaken in the faith by a strong and deep conviction, as you were taught. This is what "rooted" means.
"Abounding" means becoming ever richer in it, or growing and being built up. This is what "built up" means.
"Have perfect faith, so that there be no deficiency in it, but let it pour forth abundantly and overflow in you" (blessed Theodoret).
"Established means allowing no doubt and no wavering. 'As you were taught': you learned the faith, and you must abound in it, growing daily and acquiring a burning faith" (Oecumenius).
"He says, 'as you were taught,' so that, if nothing else, they might be ashamed before themselves. In this the Apostle laid, as it were, the foundation. In 'abounding' he points to the building.
"One must not change the teaching first received, but make progress in it. One should count it an honor to show something overflowing in one's faith, giving thanks to God because He has deemed us worthy of such grace, and not ascribing such progress to ourselves" (blessed Theophylact).
The words "abounding in it with thanksgiving" may be understood thus: abounding in faith with thanksgiving because God has deemed us worthy of such grace and has brought us near to Himself, though we were so far away (cf. Oecumenius). This is evident in all the passages cited above.
But they may also be understood thus: abounding, together with faith, in thanksgiving. The Greek text allows this. Saint Chrysostom says: "'Abounding in it with thanksgiving,' for this is what right-minded people do. I do not say simply to give thanks, but with great abundance and great zeal."
Thanksgiving presupposes the tasting of the fruits of faith and, therefore, a higher state of life within the realm of faith.