You're correct, up to a point; not everyone who claims to know the truth about God is actually right. Instead, they may be like the false prophets of Jeremiah's day: "They fill you with false hopes. They speak visions from their own minds" (Jeremiah 23:16).
But does this mean we can never know anything about God? No, it doesn't -- and the reason is because God has deliberately shown Himself to us, if we only know where to look. One way He has done this is through the world He created. As you look into the heavens... or peer through the lens of a microscope... or hold a newborn baby in your arms... can you honestly say that all this must have happened by chance? As the Bible says, "The heavens declare the glory of God; the skies proclaim the work of his hands" (Psalm 19:1).
But God has shown Himself to us in another way that almost staggers our imagination: He came down from heaven and became a man! That man was Jesus Christ, who was both fully man and fully God. Do you want to know what God is like? Look at Jesus as He is found in the pages of the New Testament -- and when you do, you'll not only realize that God exists, but that He loves you and wants you to know Him and spend eternity with Him.
Don't be captive to doubt, but commit your life to Jesus. "Then you will know the truth, and the truth will set you free" (John 8:32).