I was once one of these deniers, and I am certain that among a great many other doctrines I remain opposed because of pride rather than careful consideration and scrutiny. Nonetheless, God's eternal decree has become to me one of the most precious doctrines of Scripture. Why? What makes them so important to one's faith anyway?
The Westminster Confession of Faith answers this question for us in Section VIII of Chapter three:
The doctrine of this high mystery of predestination is to be handled with special prudence and care, that men, attending the will of God revealed in his Word, and yielding obedience thereunto, may, from the certainty of their effectual vocation, be assured of their eternal election. So shall this doctrine afford matter of praise, reverence, and admiration of God; and of humility, diligence, and abundant consolation to all that sincerely obey the gospel.
Knowledge of God's eternal decree is the assurance of one's identity. History changes, emotions change, thoughts change, life changes, but the decree of God is determined from before the beginning. Not because of what we do or contingent upon any ability of our own, but because of who we are by His Sovereign choice.
Knowing this gives us the confidence to be what we are: children of God. The epistles are full of statements that say we are holy, that we are crucified with Christ, that we are new creations, but most of our experience in this world reveals to us that we do not live up to these statements. How are we to know if we are indeed the elect? There is assurance to those who believe and obey. How do we obey? It is by God eternal decree that we obey. Even our obedience is contingent upon God's Sovereign election, for it is He who has chosen, He who has saved, justified, sanctified, and glorified from before the foundation of the world.
When the believer stumbles into sin he has ammunition against the flesh and the world to declare: "I am held fast by the eternal decree of God that I am not bound by sin, nor are my members the master of my destiny, but I am wholly His who has called me to Himself and will provide for me all that is necessary to persevere in Him."
The opening verses of 2 Peter 1:3-9 are so beautiful:
His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped the corruption that is in this world because of sinful desire. For this very reason make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For is these qualities are yours and increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins.
It is God's divine power, that is, His Sovereign Will effectively purposed and revealed in His divine decrees that provides the believer with the confidence to know with certainty that he is a child of God, freed from the dominion of sin, called into the Kingdom of God to live in righteous obedience in the worship of God to the purposed end of His glory.
Clark continues by saying, "Aside from the fact that God has commanded his servants to preach all his revelation, one great reason for preaching on the eternal decree is that a knowledge of sovereignty, election, and predestination is necessary in order to understand many other doctrines" (47). He goes on to list the perseverance of the saints, effectual calling, the necessity and nature of regeneration, the gifts of saving faith, and "in short the whole gospel" (47). There is no doubt that without the eternal decree the best hope we have is in a God whose power is limited in the same manner in which our power is limited. His omniscience and omnipotence require that the eternal decrees exist and the certainty of the eternal decrees require the acknowledgment of God's omniscience and omnipotence.
To deny the certainty of the eternal decree is to deny the immutability, omniscience, and omnipotence of God. In short, it is to deny God's very nature and being. To affirm the eternal decree is to acknowledge the immaculate Sovereignty of God whose glory deserves all of our worship, devotion, and praise--and these are accomplished chiefly through the knowledge of His Word illumined by the light of His grace afforded by the Holy Spirit and faithful obedience to the knowledge we possess in Him by the power of His Spirit and through the ministering work of His Church.