What is more important, prayer or grace?
God has desired that in spiritual combat we might fight with prayers than with our own strength… God does not command the impossible, but when he commands, he orders you to do what you can and to ask for what you cannot do on your own in order that he help you to do what you cannot accomplish on your own… Join yourself to the eternity of God, and together with him wait for those things which are beneath you. Because when your heart cleaves to God, all mortal things are beneath you. God gives help so that the command becomes possible… He does not abandon us unless we abandon him first.
from St. Augustine Answers 101 Questions on Prayer by Fr. Ermatinger.
To understand, the foundation must be established clearly that all of creation, all that was, is, and shall be, is grace of one mode or means. It is all a free gift of God; freely given, freely still given, and freely given in the future. A created thing does not bring itself into existence, does not sustain itself in existence, and does not cause its future existence. The core of sin, then, is the attempt to do any of this; to attempt to be independent from God and His free gifts of grace. This is the impossible that God does not command; for God is faithful to Himself, even if we are not faithful to Him; that is He created us in grace, to be sustained in grace, to be joined to His eternity via grace.
The commands that He has lovingly given to us, His creation, are the commands to abide in Him, to allow Him to provide for us, and to utilize that which He gives us to, through humility and obedience, cleave more to Him. They are not the impossible commands to be our own gods, but to let Him be our God, and by doing so, for us to enter into a participation in His own life.
All things become possible for those who abide in grace, for those who have the Holy Spirit dwelling within them, acting in concert with the Son, to accomplish the will of the Father for His glorification. They are possible for what God wills, He does. It is not that we can will to do anything, but that we (finally) will to be, do, and become, only that which God wills.
And this, on our end, is accomplished by prayer, which in its origins, turnings, conversions, and accomplishments is but grace and grace crowned with grace.