The word of God is compared to a mirror

"And for this reason we too give thanks to God unceasingly, that, in receiving the word of God from hearing us, you received not a human word but, as it truly is, the word of God, which is now at work in you who believe."  (1 Thessalonians 2: 13)

How is the word of God at work in us who believe?  First, you hear or read the word through preaching, teaching or bible study.  James in his Letter compares the word of God to a mirror:
"For if anyone is a hearer of the word and not a doer, he is like a man who looks at his own face in a mirror. He sees himself, then goes off and promptly forgets what he looked like."  (James 1: 22 - 23)
A mirror is a necessary tool we use to ensure that all is well with our  appearance before we we head to the door. But if a mirror shows a smear on the cheek and you do nothing about it before going out, you have not helped yourself.

The word of God becomes effectual in us when we hear the word, understands it and adjusts our life in line with what the word reveals.

"Now that you know these things, you will be blessed if you do them."  ( John 13: 17)
Change happens not because of what you know but because of what you do daily.