There are two very different ways to answer this, and Christians have always used both. The first is what theologians call the natural knowledge of God—the evidence written into the world itself. That there is something rather than nothing; that the universe had a beginning and is finely ordered; that human beings carry an innate sense of right and wrong and a persistent hunger for meaning—all of this points beyond itself to a Maker. Paul says God’s “eternal power and divine nature” are perceived through the things He has made, so plainly that no one is truly without excuse (Romans 1:20). “The heavens declare the glory of God” (Psalm 19:1). These pointers don’t function as an airtight mathematical proof, but they are real and weighty, and most people across history have found them persuasive.
But here Lutherans add something important, and even humbling. That natural knowledge can tell you that a God exists—powerful, lawgiving, to be reckoned with—but it cannot tell you that He is gracious, or that He forgives sinners, or His name. Reason peering at creation can arrive at a Judge; it cannot arrive at a Savior. For that, God has to speak—and He has. The decisive way we know God is real is that He stepped into history in Jesus Christ. “No one has ever seen God; the only Son… has made him known” (John 1:18). We do not merely infer God from a distance; we meet Him at the manger and the cross.
So if you are looking for certainty, do not expect it to come only from arguments, as useful as they are. Ultimately God is known the way persons are known—by self-revelation and trust, not by proof (Hebrews 11:6). The most honest path is not to demand a syllogism that compels belief, but to look where God has actually shown His face: at Christ. Start there, and the question shifts from “does a God exist?” to “who is this God who came looking for me?”
Scripture cited: Romans 1:19-20 · Psalm 19:1 · Hebrews 11:6 · John 1:18
Confessions cited: Augsburg Confession I