We think by the sweat of our brow we can earn favor with God. We can labor doing good works, but all our good works cannot forgive even a single sin. Our sweat or labor is a punishment for our sin; as the Scriptures say,
"By the sweat of your brow you will eat your food until you return to the ground, since from it you were taken; for dust you are and to dust you will return." (Genesis 3)
God seen to it, that by this sweat, no one can boast in His sight; but it's by believing in Christ and His one work, dying on the cross, that we are saved. The Scriptures say,
"In fact, according to the law of Moses, nearly everything was purified with blood. For without the shedding of blood, there is no forgiveness." (Hebrews 9)
That's the undeserved grace of God: That He bore our sins and took our punishment so that we would walk in righteousness. Our sweat then turns to tears when we realize how far and deep and wide and long the love of God is for us.
Believe and toil no more. Why work meaninglessly when you know the Way? All your toil is like a woman pushing and pushing in labor all to have a stillborn. Stop your pride and repent to Christ!