One thing that we will learn when we understand God's law is how far short we come as far as keeping it is concerned. In fact, in the measure that we understand God's testimonies, we will with the psalmist say, "I opened my mouth, and panted: for I longed for Thy commandments. Look Thou upon me, and be merciful unto me, as Thou usest to do to those that love Thy name" (Psalm 119:131, 132). For to open one's mouth and pant is to strive to obtain an essential of life, namely, oxygen. It is feeling the serious need for it.
So the child of God, who loves God's name, that is, loves God as He truly is and is revealed in His word, will gasp for mercy when he understands what God's law requires. For that he longs for God's commandments means that he longs to be able to keep those commandments because he loves God and considers His testimonies to be wonderful.
Surely we need God's mercy, because we fall so far short of our calling. When we understand God's law, we understand the punishment which we deserve for not keeping it. We understand how much we walk in hatred toward God rather than in love to Him. We understand that what we formerly called little sins, or, perhaps, mistakes rather than sins, were desperately wicked deeds in God's eyes, works of hatred. The more we understand God's law, the more the light of that law will expose the blackness of our souls and need for God's mercy to save us.
There is, however, more to remember. We need God's grace to supply us with strength to keep His law. In fact the word the psalmist uses is better translated as grace instead of mercy. It takes God's grace, and it was in God's grace that He came to fallen Adam and Eve, who were trying to hide their sin by their own works, to assure them that He would put enmity in them against Satan and sin. In His grace He was going to make them love His name.
Let us with the psalmist then sing:
I thirst for Thy commandments, Lord,
And for Thy mercy press my claim;
O look on me, and show the grace
Displayed to all who love Thy name.
… To show that neither His covenant nor Himself had changed, God had His apostle proclaim in the first sermon preached to the church of the New Testament: "For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." Infant baptism is based on this truth of the covenant. Because our children are included by God in His covenant, God demands that we rear them in His fear. – David Engelsma