One Door and Only One

When I was growing up in Sunday School we used to sing a chorus that went:
One door and only one
And yet its sides are two
Inside and outside
And which side are you?
One door and only one
And yet its sides are two
I'm on the inside
And which side are you?
The door to the courtyard was the only way in and it speaks so clearly of the fact that there is only one way to heaven. Even as you went through that door you had to go by the altar and to get to heaven you must go by the cross.