A Christian should understand that his mind is always under attack. Satan attacks through temptations. Added to these are the words of criticism, discouragement and ridicule of Christian and Non-Christian friends who oppose your close walk with God.
So it is your primary duty to guard your mind and heart. Imagine your mind as a fort. If there are no guards outside it is easy for enemies to get in easily and conquer the fort. That is why every fort in the world have guards. So one of the primary tasks of a Christian is to guard his mind against invading thoughts. That is what Solomon advised when he wrote: “Above all else, guard your heart, for it is the wellspring of life” (Proverbs 4:23 NIV).
What happens when a wellspring is polluted? The entire water gets polluted. Whoever drinks of it is also affected with diseases. The wellspring that should have been a source of life and health brings disease and death when it is not guarded against polluting elements.
And God’s Word identifies one of the polluting elements as anxiety or worry. We are much anxious about making a decent living in a money-prestige-power-mad world. This kind of anxiety present within our heart springs up strongly in our conversations and comes out as negative, defeating words of distrust in God. But instead make all your genuine needs known to God with thanksgiving. Then God will send a guard to keep your minds unpolluted. It is the peace of God that is beyond our understanding that shall guard your mind and heart! (Ref. Philippians 4:6,7).