Summary: We know by God by Jesus Christ because He is God. It is not enough to know about God, we must know Him personally. Fear, a healthy respect for God, is the beginning of knowledge
When Paul was writing to the church at Colosse, there was a philosophy that was beginning to emerge called Gnosticism. If you have an idea of what Gnosticism is, then you will realize that what Paul said was not only Christ honoring, but his words were very clear, sharp, and penetrating to the Colossians and what they were exposed to.
Gnosis is the Greek word from which we get our word know today. The Gnostics felt they had a special insight and knowledge of God and His secret working. They were dualists, meaning they held to a dual nature of the cosmos, of the world and the universe. They believed that the cosmos consists of both the spiritual and the material, which of course we believe also. We are not materialists who believe that the matter just organized itself by mechanical reactions that are innate in matter. We believe that God who is spirit and immaterial created matter and the universe. The Gnostics also believe this. The Gnostics got into trouble very quickly with how they believed God created the world.
The Gnostics gave it their best guess that if God who is spirit is good, then matter, which is the opposite of God, must be evil. They postulated that if God is good, then since matter is not God, then matter is not good. The question that presented itself to them was, "How could a good God create an evil world?" Their solution was to suggest that God created a being who, since he was not God, was necessarily less than God. This being created another being who therefore less than him. This process of creation goes on infinitely until by some infinitesimal calculus a creature was so far removed from God that he was able to create the world, which was matter, the opposite of God and evil. When you take the time to check out Gnosticism, you will find that there was a lot of variation in Gnostic thought as it was developed by various proponents and systems. When people tried to think of Christ in terms of Gnostic principles, Jesus and Sophia (the Greek word for wisdom) became responsible for the creation of evil matter. By Jesus, the world that came into existence, could be redeemed by enlightenment that came from secret practices. Gnostic thought tried to co-opt Christ and redefine Him in philosophical, mystical, and mythological terms. Christ became a sterilized abstraction that you could use to build your own religion to manipulate people. Gnosticism became a distraction to be consumed with the flesh and to mistake any sense of enlightenment with being on the way to escape this present evil world. Instead of Jesus being God supreme, He became the "low man on the totem pole" at the polar extreme opposite of God.
It would take quite a while to explore the effect of Gnosticism, but let me share with you what it leads to from some modern day Gnostics:
Gnostics do not look to salvation from sin (original or other), but rather from the ignorance of which sin is a consequence. Ignorance -- whereby is meant ignorance of spiritual realities -- is dispelled only by Gnosis, and the decisive revelation of Gnosis is brought by the Messengers of Light, especially by Christ, the Logos of the True God. It is not by His suffering and death but by His life of teaching and His establishing of mysteries that Christ has performed His work of salvation.