1 John 1:7
 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. (ESV)
Light and darkness. These are used by John to compare and contrast righteousness and sinfulness. Therefore, walking in the light would be walking in righteousness. It is important to note that this ability to walk in the light is something that can only happen to those who have been transformed by the saving blood of Jesus Christ. In our own ability all that we desire and all that we do is to walk in darkness.
2 Corinthians 5:17
 Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come. (ESV)
These false teachers that had infiltrated the church were claiming to walk in the light, but they had not become a new creation in Christ. They could not even know Christ because they denied his deity and were looking to "special knowledge" for their source of light. Anyone who follows any sort of "light" other than the true light that is God is simply walking in darkness still.
There is something for us here. How many people do you know who go to church regularly, give money to various charities and mission works, and yet they are depending on their own light to guide them? I can assure you there are churches filled with such people. And the sad reality is that there are people who will spend eternity in hell because they followed their own "light," which is no light at all.
Now, if we walk in God's pure light there are some wonderful results. Our relationship with one another is right. And we are continually experiencing the forgiveness for our sins by Jesus Christ. If you want to know if a church is walking in the light observe their relationship with one another, and take note if there is an emphasis on the need for the continual forgiveness of Jesus Christ.
Are you walking in the light? The only light that will guide you is God's light. You can only walk in that light if you have received the gift of forgiveness purchased by the blood of Jesus Christ. Receive it! And walk in the light!