A key question to ask is whether truth matters. Should you believe something that is false? The clear answer is "no." If it is true that God exists, and you do not believe in Him, then you are believing a false proposition. This would be a manifestly irrational position, one that would be unacceptable for every other aspect of life. It makes no sense to claim agnosticism when confronted with a disjunction that admits of no alternative; either God exists, or He does not. For one who is genuinely seeking truth, the most paramount question in life is whether God exists. And there are many good ways to know that God does exist.
Why should I believe in God?
When we say something is true, we mean that our thoughts, words, and propositions correspond to reality. If we say that which is, we speak truly. To deny objective truth is a contradiction. We would have to affirm an objective truth that there is no objective truth. Further, to say something is true means that we perceive things in the world as they are. When a seven-foot-tall person sees a basketball goal and a five-foot-tall person sees a basketball goal, they see the exact same thing from different positions. The goal itself is not changed because of the viewing position of each person. Truth does not change based on one's perspective; the object is either there or it is not, it has certain features or it does not. The way we experience something might be unique. For example, if there was only one person in the world who was seven feet fall viewing a basketball goal. Yet, if we claim that all truth is based on perception, we would have to deny that the shorter and the taller person both saw the same basketball goal. Rationality, and basic physics, quickly put an end to such a notion. Yet, when it comes to matters of a metaphysical or religious nature, the theistic skeptic, agnostic, or pluralist each cling to subjective truth. Once the impossibility of denying objective truth is clear, one can proceed to a genuine investigation of God's existence. Each person can use their objective experience of the world to understand that God exists.
Romans 1:18–20 tells us that God's existence and (many) divine attributes can be known from the things He has made. The apostle Paul tells us that God is known by all, regardless of special revelation (the Bible). Many philosophers, scientists, and other thinkers throughout the ages have concluded, independent of the Bible, that God exists. Although their conception of God was often mixed with certain (sometimes significant) errors, these men and women concluded there must be a Creator and Sustainer of the universe. The Bible reveals to us specifically that which nature reveals generally. We learn about who God is, sin, redemption, and so much more within the pages of Scripture. Through observations, reasoning, and logical conclusions based on effects in nature, we can know that God exists and that He has certain attributes (eternality, immutability, etc.). We put natural and special revelation together so that we have as complete an understanding as we can, in this life, about God.
Consider the existence of the device on which you are reading this article. Ask yourself "why does it exist?" Specifically, the question is not "how did it get here?" The question about why something exists has to do with why it exists right now. What is the reason your device (computer, tablet, phone, etc.) exists at this very moment? The answer cannot have anything to do with the manufacturer, the delivery driver, your bringing it with you to work, etc. One might venture that the reason your device exists right now is because it is composed of small bits of matter that are held together in a certain shape. True enough. But what is the reason those bits of matter hold their shape? Why not another shape? Why do they not just randomly disintegrate? There must be a sufficient reason why the matter holds the shape that it does once it is formed. For, if we say that there is no sufficient reason why the device holds together as it does, then the entire basis for even our most basic inquiry is unintelligible. We would be forced to say that the reason any particular thing exists, or an entire ensemble of things, is that there is ultimately no reason for it.
Perhaps the reason your device holds together is that there are fundamental laws of physics. Yet, we must then ask whether there is a sufficient reason for these laws? Do they explain themselves? Physical laws essentially describe how things work that already exist, so there is no way to maintain that the laws explain themselves. So, what about these other things? Do the fundamental building blocks of the universe have a sufficient reason for why they exist and function as they do? Again, if they have no sufficient reason, then that which we draw upon for explanation has no explanation.
That which exists but could in principle not exist, and that which exists but could have been otherwise, demands an explanation if we are to put any faith at all in reason. And if we abandon rationality, we might as well give up the whole game altogether. Even the question at the top of this article makes no sense to ask. We must then say that something either exists by reason of its own nature (e.g. it is necessary) or it exists by reason of something outside of itself (e.g. it is contingent). In the case of the device on which you are reading this, and any other contingent object or ensemble of contingent objects, there must be something outside of itself that explains its existence. Something that is contingent relies on circumstances outside of itself to exist.
Given what was said above, your contingently existing reading device must have a reason outside of itself for why it exists right now. And we also see that the fundamental matter, particles, laws, and so forth that the device ultimately relies upon are also contingent. We are then stuck with a futile infinite search for an explanation for contingent objects, like the device, which leaves us with absurdity. If we accept an explanatory regress or eventual fundamental "brute fact," we abandon the principle of sufficient reason which guides all inquiry. But, if there was something that existed necessarily, something that existed by reason of itself, we would have an explanation and cause of contingent objects. We would have a sufficient reason why your reading device exists right now. That which exists necessarily, who exists by reason of Himself, is God.
There can be only one necessary being, because to be necessary requires attributes and a nature that do not admit of multiplicity. And the attributes of a necessary being also entail that He is all-powerful, all-good, eternal, immaterial, and omniscient. This is but one of many reasons we can know and demonstrate God exists.
You should believe in God because His existence is the only way an inquiry like "why should I believe in God" makes sense. You should believe in God because atheism is false. You should believe in God because He loves you and desires an eternal relationship with you (John 3:16; 2 Corinthians 5:21).
Is there a God?
Can the existence of God be proven?
Why should we care if God exists?
What is truth?
Why does God require faith?
Truth about Worldview and Apologetics