I was told by two respected Roshai Yeshiva that one who isn't certain of God's existence (an agnostic) is not deemed to be a kofer (denier of His existence) as long as he thinks that His existence is probable. So one who is 51% sure of God's existence is not a kofer, but one who is 49% sure of it is. They both admitted that they had no source for this, but thought it was logical.
Can anyone provide a source for this? Are there other views? Does one have to be 100% certain of God's existence to not be considered a kofer? Is someone who isn't convinced that God exists, but is a fully practicing Jew because of Pascal's wager considered a kofer?