There have been several different attempts made throughout history to answer this question from various angles. And within the angles themselves there are additional angles, some of which were better at explaining God’s existence than others. For instance, the Cosmological Argument essentially proffers a naturalistic explanation for God’s existence by looking at the cosmos and then determining, based on propositions built off of each other, that there must be a God that exists. Hence, the Kalam argument, for example, proposes that the universe either had a beginning, or that it did not. If it did not, then one must accept that the universe is eternal, which modern astronomers and physicists are discovering is not the case.
Therefore, since the universe had a beginning, then it was either caused or uncaused. If it is uncaused, then one is left to explain how a universe began without a cause or what purpose there is in an uncaused universe. If the universe was caused, then one must explain whether the cause was personal or impersonal. If the universe was caused by an impersonal something or someone, then one is left with the problem of trying to explain how an impersonal cause could cause anything, including itself. Conversely, if the cause was personal, then one is left to explain just who or what this personal cause is. To the Christian, such an answer is far from difficult, given that the Almighty Cause has gracious confessed to His part in bringing all things to be.
The Medieval theologian Thomas Aquinas offered a five-fold cosmological explanation for the existence of God as well. To him God’s existence could be naturalistically proved through motion, efficient cause, necessity, gradation unto ultimate being, and governance of the world. By motion he argued that whatever exists and moves in creation ultimately has a first mover that set everything else into motion. In other words, all motion is caused by something or someone, and by tracing those finite movements back in time one eventually arrives at the Un-Moved Mover, who is God himself. This led Aquinas to postulate that God was also the efficient cause of nature, since all effects are caused by something or someone preceding them. And since causes and effects, by their finite nature, cannot recede into infinity, but must have an ultimate cause who initiated the first effect, then Aquinas concluded that that Cause could only be God. Third, Aquinas argued that in nature there is possibility and necessity when it comes to being. Similar to his cause and effect argument, he basically contended that things in nature have the possibility or potentiality to exist, but that that did not necessitate that they need to exist. There is only one necessity that actually exists, who causes those things with the potentiality to exist, to be, and that One necessity is none other than God himself. Fourth, Aquinas argued for the quality of the gradation of things back to the maximum example of what those things truly were supposed to be. He wrote, “Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum of heat, is the cause of all hot things. Therefore there must also be something which is to all things the cause of their being, goodness, and every other perfection; and this we call God.”1 Lastly, Aquinas asserted that there was a natural design to all that exists. That existence itself did not come into being “fortuitously” or without intelligent intervention. His claim, therefore, was that this intelligent designer was none other than whom we would call God.
From Aquinas’ final argument one can easily move to perhaps the most compelling argument being postulated today in defense of the position that God exists, and that is simply the Intelligent Design argument, or I. D. Although most secular scientists and skeptics scoff over the arguments presented by I. D. scientists and philosophers, claiming that I. D. is nothing more than rehashed Scientific Creationism of a couple of decades ago, such scoffing is born more out of ignorance and fear than reality. For although Scientific Creationism did help people see just where the battle lines were drawn between science and faith, it lacked the academic strength necessary to make headway in secular institution of higher learning. Those behind the Intelligent Design Movement not only have all of the academic qualifications necessary to take on secular academia, many are also committed Christians intent on upholding the fundamental principles of the Christian faith. Therefore, the I. D. movement has taken Aquinas’ naturalistic argument regarding an Intelligent Designer to the next level of scientific and philosophical sophistication, claiming that the natural order demonstrates extreme complexity and design which only an Intelligent Designer could provide. Perhaps the one weakness in the I. D. argument is that although it may point to an Intelligent Designer, it does not necessarily specific just who or what that designer is. It has left that determination to be argued out among the theologians.
The naturalistic explanation, though, is quite biblical for explaining God’s existence. Another term for it would be General Revelation, which Grudem defines as “The knowledge of God’s existence, character, and moral law that comes through creation to all humanity.”2 From the Bible we read,
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because that which is know about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse (Rom. 1:18-20).
From this one can gather that God has not only objectively revealed himself in nature, but has instilled that same knowledge in each and every person. Such knowledge is not enough to save or redeem anyone. It is merely knowledge that is graciously and mercifully revealed to humanity, God’s most grand creatures, for the purpose of holding humanity accountable to God for its actions, or for the glorification of God by those who understand and appreciate the general revelation given. Therefore, from a naturalistic point-of-view, there is no such thing as a genuine atheist, given that God has even revealed himself to those claiming that God does not exist. It is mere intellectual ignorance of that revelation which leads the “atheist” to make the foolish claim that he does (Ps. 14:1; 53:1).
Conversely, God has not only revealed himself objectively in the natural world, God has revealed himself subjectively through “special revelation,” to those whom God has regenerated to receive it. Again, Grudem defines special revelation as, “God’s words addressed to specific people, including the words of the Bible. This is to be distinguished from general revelation, which is given to all people generally.”3 Lewis Sperry Chafer added along with the Bible that God is revealed through providence, preservation, miracles, direct communication, and ultimately in the incarnation of Jesus Christ. He said, “God could not draw nearer, nor could He disclose more clearly the wonders of His Person, the perfections of His purpose, nor the depths of His love and grace, than He has done in the incarnation, which in the scope of its purpose embraced the life, teachings, example, death, and resurrection of the eternal Son, the Second Person of the Godhead.”4 Yet, since Jesus is no longer walking the face of the earth, and since Jesus has sent God’s Spirit to bear testimony of the person and words of Jesus, which are only recorded in the Bible, then in order for a person to benefit from God’s special revelation of himself in the person of Jesus is going to require a regeneration of a person’s being through the Holy Spirit of God.
So, to answer the question of how one knows whether or not God exists, there are actually two answers that could be given. One, objectively, God has revealed himself in nature, and that revelation has been instilled in every creature that God has created in his image. Nature has intentional structure and design to it simply because God intentionally structured and designed it. Two, subjectively, a person can know of God’s existence more specially or specifically through the pages of the Bible, but only after God decides to draw that person into a special relationship with himself via a regenerative act of the Holy Spirit. Until then, the words recorded in the Bible are going to be mere garbled words at best; a convoluted, contradictory, irrelevant tome at worst. Whatever the case, God has not left creation without a witness as to his existence. It is just a matter of whether or not the creature accepts or ignores God’s revelation, both of which God plays a part in either the further illumination or dulling of the spiritual sight of those whom God has chosen to see or to remain blind.