The word "for" (here meaning "because") should send us immediately looking back to the previous verse to connect the verses and discern the meaning. No one should "take us captive through deception" because the fullness of the Divine nature dwells in Christ Jesus. There is no need to look elsewhere for answers or salvation. He is sufficient for all we need. Any earthly philosophy or theology that draws us from the full reliance on Him and the grace he daily bestows upon us should be refuted. Only He is worthy of worship and praise.
Many speculate that the Colossian heresy may have been pre-Gnostic and therefore have included thoughts that Jesus was not really a flesh and blood man. It is admittedly mind blowing to believe that Jesus was no less human than divine and no less divine than human. He was 100 percent both. Jesus was not a mirage, spirit, or phantom. No one saw him and yelled, "AHHHH! A GHOST!!!" Being fully human He ate, drank, slept, sweat, walked, and talked like any other man would. Being fully God He healed, raised from the dead, taught with authority, and pierced to the heart of the matter as only one who was divine could do.
In Him we have been made complete. We need not look to the world for our completion. Relationships will fail. Our friends may forsake us. Our spouses may let us down. Our children may avoid us. If we seek wisdom, we must look to Him. If we seek true love, we must look to Him. If we seek joy, peace, patience, kindness, goodness, gentleness... well, you get my point. Human relationships will ultimately fail us at least in part. Our identity and completeness is found in Him. No politician can save us, no human love can complete us. Only in Him are we made complete.
All fullness dwells in Christ—that is to say, it is never exhausted nor diminished. On the last day wherein this world shall stand before it is given up to be devoured with fervent heat, there shall be found as much fullness in Christ as in the hour when the first sinner looked unto him and was lightened.
- C.H. Spurgeon