Jesus had told the Disciples that their 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 Jesus the Christ and the Kingdom of God. But when and where did Peter use the keys? And what were the spiritual "doors"? To find out it's necessary to track him.
Peter's path of Kingdom ministry began, together with the other apostles, with some primary preparation.
First of all Jesus "... breathed on them, and said to them, 'Receive the Holy Spirit" (John 20'22). He did this so that they could receive His Kingdom teachings.
Acts 1'3 indicates that:
"He also presented Himself alive after His suffering by many infallible proofs, being seen by them during forty days and speaking of the things pertaining to the kingdom of God".
Straight after the ascension of Jesus but before the Holy Spirit came on the Day of Pentecost the first "door" that would require unlocking faced Peter.