GK Chesterton once said that an open mind was like an open mouth... it was intended to be closed over something solid. What he meant was the purpose of an open mouth was to close it around food and the purpose of an open mind was to close it around truth.
You don't want to your mouth to admit disease or poison into your body and you don't want your mind to permit error into your mind. So the concept of the "open mind" is not accepting any idea as valid as any other, but assessing each idea to see if it is true or not.
Modern society seems to make two errors when it comes to an open mind. One is incredulity. The other is speculation.
To be incredulous is to be "unwilling or unable to believe something." To be speculative is to be "engaged in, expressing, or based on conjecture rather than knowledge."
Neither behavior indicates an open mind. The incredulous person refuses to consider whether a thing is true. The one who speculates does not give enough consideration before accepting a thing as true. They aren't opposites however. One can be incredulous because of a speculation they have previously formed.
The open minded person, in contrast to the incredulous or speculative types, seeks to learn what they can about what is true. He or she recognizes when his or her knowledge is lacking and does not think this lack of knowledge means that the idea can just be accepted or rejected. "I don't know" means "I must learn more" to the open minded person.
Open minded thinking doesn't mean never reaching truth. Rather, it means that once we recognize something is true, we're no longer free to accept error on that subject. Once we realize [X] is true, all other considerations which revolve around [X] must recognize that truth.
Unfortunately, the incredulous person begins with the assumption that [X] is impossible and therefore can never happen -- he or she thus refuses to consider any theory that argues [X].
The problem is, many people simply hold "X is false" based on conjecture and assumption. While one can reject something based on reason (for example, identical twins having drastically different levels of happiness show a fatal flaw with astrology), many simply hold their assumptions without questioning if they are true.
Let's consider the concept of life in places other than Earth. Personally, I'm agnostic on the subject. An argument based on the huge number of star systems claiming such life must exist is speculation. But on the other hand, it's foolish to claim such life can't exist because we haven't found it yet. That's incredulity. We can't know it does exist unless we find it, but we can't know it doesn't exist unless we explore every planet in the universe. The only open minded approach is to say, "I don't know, but I will consider credible evidence if it appears."
Some might wonder if the above example would justify agnosticism in considering the existence of God. I would say "not really." If aliens exist, that is a matter of physical existence and physical proof. But the concept of God is supernatural. Literally "above nature." You can't use science (by nature aimed at the physical universe) to prove the existence of the supernatural -- that's like expecting a microscope to prove astronomy.
Aha! you might say. "Without physical proof, it means you can't prove the existence of God, but can only speculate!"
To which I reply, "Prove you love your spouse or child." See, things exist that do not have a material existence we can scientifically study. You can say you love someone, that you are thinking a thought, but if the only proof that exists is physical proof, then only things with physical existence can be proven. If only things which can be physically be studied exist, then none of our thinking, reasoning, etc. exist.
The thing is, despite the claims of 'freethinkers,' the denial of Christianity is not an open-minded act of rationality. It is incredulity formed by speculation, a refusal based on a too hasty assumption made without proof.
