You must imagine that a human beings are born (those that don't die in child birth) into a confusing and terrible world. Many die in infancy, others are afflicted with suffering. They have appendixes and other unnecessary appendages. This means a designer who thought of doing things in such a way is a very cruel, mean designer.
You must believe that God chose to not reveal to a great number of people across the world and across time that they need to believe in his son in order to be saved. This also is a cruel position to take.