If you are a Christian, it matters because scripture tells you to go out into the world and spread the gospel.
I can't tell you why it would matter to an atheist, or to an agnostic, because I don't see it from that point of view.
I rarely get on this forum because it seems everyone is spoiling for a fight, and they try to prove their point, or ridicule the opposing point to make themselves feel better about what they have chosen to believe.
Religion is not something you learn, just as you can't teach someone to love, you can't teach someone to have a relationship with God. Studying scriptures, prayer, fasting, etc. may help you to improve your understanding, but it can never lead to salvation.
I am a Christian, but I don't try to force my beliefs on anyone. If I get the opportunity, I will mention it, and if there is interest, I will discuss it, but the Bible says that salvation has to be personal, it can't be forced or taught.
Unfortunately history is littered with people who have tried to use religion as a weapon. It appeals to the sociopath and the demented, because if you can convince others that you are sent by God, no one has more power than you. Many atrocities have been comitted in the name of God, using religion as an excuse to do what they want. The Bible says that you will know them by their fruits, you can claim to be a Christian, but if you are being judgemental, spiteful, manipulative, and proud... you are not a Christian. Christ set the example. When he walked this earth, his friends were not the religious people of his day, he chose to assosciate himself with the prostitues, the homeless, the tax collectors, the fishermen... he called those that claimed to be superior and righteous, tombs. He said on the outside they were white and clean, but in the inside they were full of decomposition and bones.
Matthew 23:27 (New Century Version)
"How terrible for you, teachers of the law and Pharisees! You are hypocrites! You are like tombs that are painted white. Outside, those tombs look fine, but inside, they are full of the bones of dead people and all kinds of unclean things.
I think too many times Christians, and religious people in general start acting like Pharisees. They become so enamored in their own self-rightouesness that they start looking down on those they feel are ignorant.
I'm sure the same thing applies to atheists/agnostics. They become so convinced in the superiority of their arguement, that the feel that have to attack the opposition to prove their point.
In the end, everyone gets defensive and the arguements spiral out of control. I think that it's possible, but very difficult, to have peaceful and open discussions about both sides differing views. Religion is a very polarizing subject, and it generates strong feelings from both sides. If anyone wants to have calm and respectful discussion on the subject, I will welcome it. I think there is much that both sides can learn from the other, but there has to be respect and understanding for this to work.