Wouldn't it be nice if we could make ourselves pure simply by eating the right things? Granted, it sometimes is hard to eat the right things. But it's a lot harder for most of us to say and do the right things.
We have all sorts of problems with our speech: We say too much, we gossip, we lie, we take the Lord's name in vain, we exaggerate, we distort, we slander, we wound, we curse, we are crude, we... The list could go on and on because the tongue can get away from us. Part of the problem is that our mouths often let fly what is unredeemed in our hearts. Another part of the problem is that we don't think about what we're going to say before we say it. Still another part is that we do think what we would say to wound, hurt, or get back at someone; then, when we are given the opportunity, we say it.
Purity is an ongoing struggle. Jesus is saying don't make it so simple, so superficial, as just a matter of what you eat — or just "going to church," or just reading the Bible, or just... Appearances are important because others should see Jesus when they see us, but we also need to remember that appearances by themselves are not enough. Our thoughts, words, and actions need to come from a heart committed to Jesus (Ephesians 4:22-24, 6:24) and not worried about measuring up to a human list of religious external practices.
Holy God, forgive me for wanting a cheap, quick fix to the problems of my heart's purity. I want you to redeem my heart. So I commit today to avoid what can pollute, putrefy, and poison my heart toward you and your children, and I ask for the power of the Spirit to help me with this commitment. In Jesus' name, I pray. Amen.