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The Gnostics claimed to know more than anybody else; Gnostic means "to know." Gnosticism was the process, or system, of knowing more than anybody else, particularly the "mysteries." However, the word that Paul uses here, translated "knowledge," is epignosis, which means "complete knowledge."
The apostle is saying, then, "Brethren, you already have complete knowledge. These people cannot add anything to you." He is letting them know that they had gone backwards, giving up the riches of the knowledge of God for nothing more than bread crumbs by comparison. By writing in Colossians 2:8, "Beware lest anyone cheat you through philosophy. . . ," he warns them not to allow themselves to be taken captive, like the spoils of war—spiritual war. He did not want them taken captive and enslaved to demon worship through a philosophy that might appear to be enhancing their relationship with God but was really a perversion of the truth.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 21)