What We Believe Doesn't Affect What Is
Many people think the existence of God is a matter of personal belief (you just gotta believe!). So they either believe or they don’t, based on their faith, or lack of faith, or blind faith, or blind doubt. The concept of God as the Absolute Truth is not commonly understood.
For example, some don’t believe in God, but fervently believe in the right to drinkable water. Others believe 100% in free enterprise—clean water (and liberal sentiments) be damned. Without reference to an Absolute Truth, though, who’s “right?” What makes one opinion any better than another?
Two plus two equals four whether we believe it or not. If I simply believe I'm a trillionaire, should your bank honor my checks? Belief or disbelief doesn’t make anything true or false. Gravity works regardless of our belief in it. My believing I can fly won’t alter the reality of my hitting the ground, hard.
If we think of God as a concept we can either believe in or not—a metaphysical entity who only exists in the mind of the believer—we’re not thinking about God at all—at least not according to the “God as the Absolute Truth” concept found in Bhagavad-gita and Srimad-Bhagavatam. Readers there find themselves surrounded by precise, meticulous explanations of Transcendental Personality unimagined by most people on opposing sides of the fence of belief.
Bhagavat literature presents personalism. Personalism means that God is a Person Who can be known to anyone—just as we know our friends, relatives, and neighbors. God’s existence doesn’t depend on anyone’s belief, any more than your or my existence does. The idea is to know God as the Absolute Truth—as the Person He is—not to “believe in God.” Believe it or not.
The Gita presents a method of knowing God which ought to be familiar to anyone. It's similar to how we get to know anybody. There’s a process for getting acquainted—which requires interest, inquiries, and introductions. In the same way, it’s possible to know—as truly as you know anyone—that Krishna (the personal name of God most commonly found in the Gita and Bhagavatam) exists.
Most people, though, just aren't interested. All that's required is to take a few simple steps, but, if you’re not really interested, you won’t even try to make the acquaintance. Meeting Krishna Himself involves a change of perspective, and most of us would prefer to go on believing what we want to believe. It's much easier to believe or disbelieve in God than it is to factually know Him.