Psalm 51:3-7, 12-13
Matthew 15:1-2, 10-14
In response the charge that his disciples aren't following the traditions that the Jewish leaders had set up, the Lord reminds them and us that it is not what enters the mouth that defiles us but that the things that come forth from our mouth do. He speaks to the reality that sin happens primarily in our hearts and then it is often manifested in words. I find it significant that we have a concrete example of this in the reading from Numbers, as Miriam and Aaron spoke against Moses. They didn't do anything, they simply spoke against him and that was enough to merit punishment from the Lord.
One way that the Church has encouraged us throughout the years to purify ourselves is by having a devotion to the Holy Name of Jesus. In the liturgy we bow our heads at the name of Jesus. In conversation we ought to use His name sparingly and if it is spoken in vain - what a great practice to say in response "Blessed be the name of the Lord!" to remind others that His name is to be held sacred. Furthermore, the scriptures tell us that at the name of Jesus, every knee bends in heaven, on the earth and under the earth. So as we use our very lips to speak that powerful name, that name which has the power to cast our demons and cure the sick, we are mindful that our lips ought to be a place of purity and praise; on them ought to be praise, uplifting words, and truth, not negativity, profanity and lies. In this way we can ensure greater purity for ourselves.