If I'm careful and diligent to guard my mind, take my thoughts captive, and mediate on truth (according to God's word)...that will show in my words. Lord willing, the light that fills me can come out and pierce through a little bit of the darkness in the world around me. A bit of the darkness in others' souls. I won't even have to try, it will happen naturally.
The mouth speaks out of the overflow of the heart.
"Let the words of my mouth and the meditation of my heart be acceptable in Your sight, O LORD, my rock and my redeemer." (Psalm 19:14 ESV)
"...whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is anything worthy of praise, think about these things." (Phil. 4:8 ESV)