At every turn, it seems, the main object of Gnosticism was to twist the nature of Christ. Some Gnostics believed that Jesus was a man, but that Christ entered into Jesus when He was baptized and left Him right before He died. Other Gnostics believed that Jesus did not really die - because, after all, if He died, then He was not really God. Others believed that He could not have been perfect and sinless because He created matter, which Gnostics believed to be evil. And there were also those who believed that Jesus Christ was a created being - an idea that is still affecting the fringes of the church of God today.
So if we want to counter Gnosticism, we must begin with the truth of Jesus Christ. Paul emphasizes this in verses 9-10: Jesus was the fullness of the divine nature in bodily form, and He is the head, the leader, the sovereign, of every principality and power. Though the Gnostics in their various views always twisted or denied some aspect of the nature and role of Jesus Christ, these truths brought out by the apostle are bedrock beliefs for true Christians.
Also foundational to countering Gnosticism is the truth that Jesus brought. To combat the false knowledge that threatens to plunder our spiritual riches, we must take the Bible as the complete and inspired Word of God, against which we can test any concept, tradition, doctrine, or philosophy, no matter how good it sounds on the surface. Gnostics would not readily accept the Bible as God's inspired revelation, or if they did, they also held that other ancient, secret writings were on par with Scripture, and could be trusted to provide greater insight.
In addition, Gnostics were also avid proponents of "progressive revelation," the belief that God is continuing to reveal His will to mankind, but with the implication that Holy Scripture is not as important as hearing directly from the spirit world. Thus, some today, while not entirely rejecting the Bible, believe that "God" is personally revealing things to them - things which often contradict what He has already given to mankind in the His written Word.