If our confessions represent the doctrine of Scripture, then “classical theism” is simply another way of saying “the biblical doctrine of God.” What is this biblical doctrine of God? Chapter 2 of the Westminster Confession of Faith states it as follows:
There is but one only, living, and true God: who is infinite in being and perfection, a most pure spirit, invisible, without body, parts, or passions, immutable, immense, eternal, incomprehensible, almighty, most wise, most holy, most free, most absolute, working all things according to the counsel of His own immutable and most righteous will, for His own glory; most loving, gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin; the rewarder of them that diligently seek Him; and withal, most just and terrible in His judgments, hating all sin, and who will by no means clear the guilty.
God hath all life, glory, goodness, blessedness, in and of Himself; and is alone in and unto Himself all-sufficient, not standing in need of any creatures which He hath made, nor deriving any glory from them, but only manifesting His own glory in, by, unto, and upon them: He is the alone fountain of all being, of whom, through whom, and to whom are all things; and hath most sovereign dominion over them, to do by them, for them, or upon them whatsoever Himself pleaseth. In His sight all things are open and manifest; His knowledge is infinite, infallible, and independent upon the creature, so as nothing is to Him contingent, or uncertain. He is most holy in all His counsels, in all His works, and in all His commands. To Him is due from angels and men, and every other creature, whatsoever worship, service, or obedience He is pleased to require of them.
In the unity of the Godhead there be three persons, of one substance, power, and eternity; God the Father, God the Son, and God the Holy Ghost. The Father is of none, neither begotten, nor proceeding: the Son is eternally begotten of the Father: the Holy Ghost eternally proceeding from the Father and the Son.
If this is the God who is, who has created all things, then the way that this understanding of God drives evangelism should be clear. He is infinitely perfect, and He is infinitely sovereign over all His creatures, and yet man has rebelled against and rejected Him. Understanding the true nature of God shows us clearly that God is different from human beings. He knows all, for example. The sinner cannot con God in the same way he might try to con a human judge. This most holy God hates all sin. This God will judge all sinners. And this God will not clear the guilty. This God is also “most loving, gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin; the rewarder of them that diligently seek Him.” He has provided the means of redemption in Jesus Christ.
This is all somewhat general, but let us focus on some of the attributes of God that are most under discussion today. Take the doctrine of God’s aseity, for example. God, according to classical theistic doctrine, is self-existent. How might knowledge of God’s self-existence be of any practical value to us in our own Christian lives or in our evangelistic efforts? In the first place, it helps us understand the Creator-creature distinction. We need to know that God does not derive His existence from anything else. The Creator is not Himself a creature. We derive our existence from God. This should give us great confidence in our evangelism. If God is the One who called all things into existence, we can be confident that He calls the spiritually dead to life.
What about the doctrine of divine simplicity, the idea that God is non-composite? How in the world does that idea encourage or motivate evangelism? In the first place, we need to grasp the significance of divine simplicity for a proper understanding of God. In several lectures and interviews, James Dolezal has asked a helpful question to indicate the importance of divine simplicity. The basic question is this: Does God require anything other than God in order to be God? If your answer is no, you are implicitly affirming the necessity of the doctrine of divine simplicity, because if God is composite, He does depend on something other than God in order to be God. Again, simplicity is an attribute of God that clearly reveals the Creator-creature distinction. God does not have a collection of attributes that are parts of Him. He is His attributes. The love that Scripture says motivated the Father’s sending the Son for us and for our salvation cannot cease to exist any more than God can cease to exist because God is His love (1 John 4:8, 16).
Finally, what about the doctrine of divine immutability, the biblical assertion that God’s being/attributes do not change? There is no attribute that is more of an encouragement to evangelism than God’s immutability because it reveals that God is not fickle. His promises will never change. When God promises that all who believe in the Lord Jesus Christ will be saved and that they will have eternal life, we can rest in that promise. God will never go back on His word because to go back on His word would mean that He would cease to be God, and that is impossible.
Christians are called to take the gospel to a lost world, but the only gospel worth taking to a lost world is the gospel involving the good news about what the God who has revealed Himself in Scripture has done for us and for our salvation. A gospel that calls people to turn to a false God is no gospel at all. Classical theism is simply a shorthand way of describing God as He has revealed Himself in His Word. There is no other God.