PETER USES THE KEYS
Jesus told the Disciples that the foremost purpose was: “Go into all the world and preach the good news to every creature," Mark 16'15.
This meant preaching the Good News of Himself and the Kingdom. So when did Peter use the keys? And what were the spiritual "doors"? To find out that it's necessary to track him.
For the first key, some preparation was required. That's because, starting with Acts chapter 1, the first "door" to face him appeared after the ascension of Jesus but before the Holy Spirit came on the Day of Pentecost.
To enable the Disciples to engage it, Jesus had done two things. "He breathed on them, and said to them, 'Receive the Holy Spirit'," John 20'22. (Note, He did this because it was necessary. But for one purpose only.)
Next was: "He through the Holy Spirit had given commandments to the apostles whom He had chosen," Acts 1'2. (Note, "through the Holy Spirit", who He had breathed into them.)
In other words He breathed the Holy Spirit into them so that they could receive His Kingdom teachings.