"Existentialism" probably needs some clarification. The formal definition is "each person exists as an individual in a purposeless universe, who must oppose the hostile environment through the exercise of their free will."
Put simply, this says that since we are "on our own" in the universe, we can believe anything that seems right to us. But does believing something make it true? For example, if someone told you to use the brake on your car instead of the accelerator because it "made sense to them," how far would you get? Obviously, you need to be told the truth, so you can use the accelerator to make your car move.
The same thing is true of Christianity. Since it is true, verifiable, and provable; we need to know the truth-not some person's opinion or interpretation. It just makes sense-if God is really God, He should know exactly what we need and how to tell us using the Bible.