Jesus is speaking of spiritual vision. If we keep our attention (eye) single upon Jesus, the Word, then all that we will be filled with is the light of His Word (Jn. 1:1-14; Ps. 119:105). The key to victory is this singleness of purpose (Phil. 3:13). The strength of the laser lies in the fact that all of the light is concentrated on one single point. Likewise, the strength of the Christian lies in how single his or her vision is upon Jesus, ‘the author and finisher of our faith.’ (Heb. 12:1-3)
You may have God and riches, but you can only serve one of them. In Matthew 6:24, the Lord amplifies this by saying that we cannot serve two masters. Why is this so? Because we do not have the capability of faithfully serving both of them. You will cleave to one more than the other. You cannot serve both with all of your heart. We just don’t have the capacity to do our best in two areas at the same time. God wants singleness of vision, singleness of purpose. He wants us to serve Him with all of our heart.
You may be wondering how you can still make a living without always thinking about money. Where will it come from? How will you get more of it? In verse 33, Jesus goes on to say that He will supply all of your need when you seek first the Kingdom of God. All of these other things (food and clothing etc.) will be added unto you. If we will simply put God and His Kingdom first in every area of our lives, He will add to us all the wealth that we need. It’s when we covet after money that we ‘pierce ourselves through with many sorrows’ (1 Tim. 6:10). Put God to the test today. Put Him first in every area of your life and see if He won’t provide all the other things for you.