“But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.”  
1John 1:7

He had just finished saying:

“If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:”  1John 1:6

Look at that standard! He would consider darkness any decision borne without fellowshipping with him. In other words, he is saying that he wants us to walk in his light and the only other option outside of that is to walk in darkness.
He wants you to replace “I think” to “the word said” in your vocabulary.
Your decisions and pattens of thoughts become influenced by the word of God.

“So mightily grew the word of God and prevailed.”  Acts 19:20

The word of God unveils to us his prevailing nature, as you spend more time with the word of God, he would prevail in your life. It wouldn’t be you struggling to always remember what you just read in the Bible so you can apply it in your life’s situation, as you spend time with the word of God, he enters your thoughts and for every choice you have to make, there is a picture from the word of God available already for your use.
He said if we walk in his light, we would never walk in darkness because he would always show up; if we spend time with the word of God, there would be no point of confusion in our lives, a picture from the word would always be plastered in our minds for our use.