why does arthur first pull the sword from the stone

Sir Kay needed a sword, and Arthur was sent to fetch it..... he was locked out of the house, so he ran back and removed the sword from the stone.

The people would not accept that Arthur would be the King. As far as they all knew he was a boy and of common birth. No one knew the King had given him up. So they told him to back again at Christmas and allowed other men to try. They all failed but Arthur came at Christmas and pulled out the sword yet again. They made him come back two other times before the officials relented realizing that Arthur was meant to pull the sword out and no other man would succeed.