When you speak the truth, you're speaking for God. When you lie, you're speaking for the devil.
If you believe that God is real, God created and sustains everything, so when your words are in alignment with God's creation - reality - it's like you're speaking on God's behalf. You are representing reality, and therefore representing God, with your words.
When you lie, you are rejecting God's creation (objective reality) and imposing a false creation of your own with your words, usually in an attempt to steal another person's connection to reality. As such, you are representing the devil.
"When the devil lies, he speaks his native language, for he is a liar and the father of lies."
-Jesus (John 8:44)
Speak the truth to each other ... declares the LORD.
Zechariah 8:16, 17
For we cannot oppose the truth, but must always stand for the truth.
2 Corinthians 13:8
The truth shall make you free. -Jesus (John 8:32)