Most of the cathedrals we saw in Rome had amazing, beautiful doors. And one was always more beautiful than the rest. "The Holy Door" is only opened during the "Holy Year", the Jubilee year, every 25 years. It's bricked up on the inside.
From: 'St. Peter's - Guide to Basilica and Square'
The last door on the right is the "Holy Door". This door is bricked up on the inside. On the first day of the Holy Year the Pope strikes the brick wall with a hammer, and so opens the door to let in the pilgrims who come to make the most of the indulgence. It will be closed by the Pope himself at the end of the Holy Year. The Holy Door represents Jesus, the Good Shepherd and the gate of the sheep pen: "I am the gate. Whoever enters through me, will be safe. He will go in and out, and find pasture" (Jn 10:9).
They have carvings telling the stories in the Bible. I could have stared at them all day. Here are a few.
Psalm 24:7 Lift up your heads, you gates; be lifted up, you ancient doors, that the King of glory may come in.
That's the verse that would pop in my head every time I saw one of those doors. I can't imagine the entrance that Christ will make when he returns. Won't need a door!