In John 10, Christ enters by the door (the appointed Jewish way) into the Jewish sheepfold, but afterward Jesus Himself becomes the only door, or entrance, into life. There is no such thing in the present day as knocking at this door, for it is always open. It will be shut, and then it will not be re-opened (Luke 13:25). An open door means a way opened up by God for His service in any particular place.