"Undefiled Mouth"
The Holy Spirit illuminated child of God knows that he has received mercy from God, therefore he wants to make Biblical sacrifices that are pleasing to God. Romans 12:1-2 gives clear instructions on how to be a living sacrifice unto the Lord. "Therefore, I urge you, brothers, in view of God's mercy, to offer your bodies as living sacrifices, holy and pleasing to God--this is your spiritual act of worship. Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is - His good, pleasing and perfect will."
The promises proclaimed through Isaiah and the whole of Scripture regarding the suffering Savior teach us that God will raise up a people who would proclaim the truths of Christ. There are three heart desires found in the child of God's life that enable him to be a witness for Jesus Christ. The first is the desire to have an undefiled mouth. Isaiah wrote of Jesus Christ, "He had done no violence nor was any deceit in His mouth." (Isaiah 53:9)
Jesus taught that speech as a whole is a reflection of who man really is. Jesus said, "Make a tree good and its fruit will be good, or make a tree bad and its fruit will be bad, for a tree is recognized by its fruit. ...For out of the overflow of the heart the mouth speaks. The good man brings good things out of the good stored up in him, and the evil man brings evil things out of the evil stored up in him. ...for by your words you will be acquitted, and by your words you will be condemned." (Matthew 12:33-37)
Speech that is predominately true, proper, chaste, instructive, and righteous proves that the heart is right before God. Speech that is often false, envious, malignant and unrighteous proves that the heart is wrong. The words that we speak as a whole show forth the fruit that is inside our heart, just as we can know the tree by the fruit it bears. A sinful heart produces sinful speech and a heart controlled by the Holy Spirit of Christ will predominately produce speech that is pleasing to the Lord.
The Holy Spirit has this question.
Does Your Speech Proclaim the Gospel of Christ?