en.wikipedia.org/wiki/Keys_of_Heaven
Saint Peter is often depicted in Roman Catholic and Eastern Orthodox paintings and other artwork as holding a key or a set of keys. The general layout of St Peter's Basilica also is roughly key-shaped; evocative of the keys entrusted to Saint Peter. Since the 16th century a symbolical pair of keys is created for every pope and buried on death with him.
en.wikipedia.org/wiki/Keys_of_the_kingdom
As found in the Gospel of Matthew, chapter 16, within the context of verses 13-20:
13 When Jesus came to the region of Caesarea Philippi, he asked his disciples, “Who do people say the Son of Man is?”
14 They replied, “Some say John the Baptist; others say Elijah; and still others, Jeremiah or one of the prophets.”
15 “But what about you?” he asked. “Who do you say I am?”
16 Simon Peter answered, “You are the Messiah, the Son of the living God.”
17 Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven. 18 And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it. 19 I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” 20 Then he ordered his disciples not to tell anyone that he was the Messiah.[4]
- (Matthew 16: 13-20) (NIV^)
en.wikipedia.org/wiki/Coats_of_arms_of_the_Holy_See_and_Vatican_City
Ecclesiastical heraldry had the same origin and developed contemporaneously with general heraldry, which had become general throughout England, France, Italy and Germany by the end of the 12th century. Ecclesiastical heraldry appears first in seals, nearly all vesica-shaped.[
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