Let me ask you a question: Do you apply this guideline to any other area of your life? For example, do you ignore physical dangers, saying it doesn't matter what risks you take "as long as you're sincere"? I doubt it.
Let me illustrate it this way. Suppose I go to my medicine cabinet and take out a bottle of liquid that I think will cure my cough. But what if I ignore the label that says the contents are dangerous if swallowed, because it's only meant to be used on my skin? I might sincerely believe it'll cure my cough -- but if I swallow it, my sincerity won't save me from harm.
In a far greater way, the same is true with God. God loves us -- and because He loves us, He has provided the way for us to be forgiven of our sins and go to be with Him in heaven forever. But what if we ignore His prescription? What if we even reject it, thinking some other way is better? We may be sincere -- but our sincerity won't save us.
But has God provided the cure we need -- the cure for the disease the Bible calls sin? Yes! That "cure" is Jesus Christ, who came from heaven to save us. This is why the Bible says, "Salvation is found in no one else, for there is no other name under heaven given to men by which we must be saved" (Acts 4:12). Don't gamble with your soul, but give your life to Jesus Christ today.