Summary: This message, based on Matthew 5:8, shows how impossible it is to have a pure heart without God’s help.
I. Purity Beyond Our Reach
Three of the ways we can stumble in purity is with impure words, impure emotions and impure thoughts
a. Impure Words James 3:9-10 (quickview)  says, “With it (the tongue) we bless our God and Father, and with it we curse men, who have been made in the likeness of God. Out of the same mouth proceed blessing and cursing. My brethren, these things ought not to be so.”
b. Impure Emotions Matthew 5:21-22 (quickview)  says, “You have heard that it was said to those of old, ‘You shall not murder, and whoever murders will be in danger of the judgment. But I say to you that whoever is angry without a cause shall be in danger of the judgment...”
c. Impure Thoughts Matthew 5:27-28 (quickview)  says, “You have heard that it was said to those of old, ‘You shall not commit adultery’. But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.”
You say these standards are too hard to obtain. They are meant to be hard (and impossible) unless we have God living in our heart. Even though we may try to say or do the right things that appear to be upright to others, this effort is in vain without a pure heart. As we seek to obey the will of God we understand that it is impossible to have a pure heart on our own strength. But with God’s help a pure heart is something all of us can have.
II. Purity of the Heart
But what is a pure heart? In 1 Timothy 1:5 (quickview)  , a pure heart is linked with a good conscience and a sincere faith. A pure heart is when God has your heart. Now the heart is the center of our emotions and will, and spiritually the heart controls our entire being. If God has your heart, he has everything else of you too. If God doesn’t have your heart, he really has nothing of you.
How do we obtain a pure heart? We must repent and ask God to forgive our sin and to forgive our lukewarm attitude toward purity. And we must invite God to take control of all of our heart. Asking God for a pure heart should be done on a daily basis as we renew our relationship with God every new day.
This is what God wants more than anything else for you and me. That’s why he said that the greatest commandment is to love the Lord your God with all your heart...
Furthermore, only God can make our heart pure and only God can judge our heart to be pure. You see it’s possible to fool all the people all the time, but you can’t fool God any of the time.
Illustration: A father and son were driving down a country road and saw a watermelon patch a little way off the highway. The father said to the boy, "Keep a lookout here while I go get a melon." He snuck into the patch, lifted a choice melon from the vine, and then called to the boy, "Is anyone coming? Look both ways." The little fellow wisely responded, "But Daddy, shouldn’t we look up too?" (Our Daily Bread , April 18, 1999)
Having a pure heart is to be so close to God that we don’t have to look up to see if he’s watching. It means we have a personal relationship with God and we only have to look within our heart to see Him.