Crossed Keys
Keys are not an uncommon bearing in heraldry. In the logo of the Swiss bank UBS for example, there are three keys.
Keys are common also in the insignia of religious houses. Most frequently, two Crossed Keys are associated with St. Peter and the keys of heaven. But why two keys? USB might have three locks on its vaults, so are there two locks on the gates of heaven?
The answer to this is in Matt. 16:18-19, which is not so easy to understand and even less easy for different Christian denominations to agree the precise meaning. For sure, Christians do agree that our sin locks us out of heaven and Christ's passion unlocks these gates and allows us to enter.
We look at this in a little more detail below, but first we'll see who is using them heraldically.
Keys are ubiquitous in the Church in Rome because of its strong association with St. Peter. But keys are not exclusively Roman Catholic; they appear, either singularly or in pairs, in a number of church logos. For example:
(Click any image to enlarge)
Papal coat of arms
Included in the papal coat of arms are two symbolic keys forming a St. Andrew's Cross. One key is silver and the other gold. These keys appear on the coat of arms of the Holy See, the coat of arms of the Vatican City, and the coat of arms of every pope since the 12th century.
An interesting example is shown on the right. It's a pectoral cross commemorating the papacy of Pius IX, the longest-reigning elected Catholic Church pope. (Photo reproduced with the kind permission of Liturgy, USA, who had offered the cross for sale on eBay. ).
Click the photo to enlarge, and at the top of the cross we see Pius' coat of arms; on the left is "IHS"; on the right we see "SMR", which presumably means "Sancta Maria Regina", since Pius had a lively devotion to the Blessed Virgin Mary; and on the reverse we see images of Peter & Paul.
By definition, pectoral crosses are made to be worn. Yet this one is unlikely to have been worn by the pope himself, since most pectoral crosses are the pope's own from before his election and would obviously lack the papal arms and the crossed keys. It's also unlikely that he'd have his selfie stamped on the front!
Therefore although this includes a quintessential symbol of the papacy, it is most likely worn now in commemoration of Pius IX. (Catholic lecturer Sean Wright suggests this particular cross was commissioned by the pope to be handed to favoured prelates.)
Meaning of the keys
The keys represent two separate functions; the power to bind and to loose on earth (represented by the silver key); and the power to bind and to loose in heaven (the gold key). These are a reference to St. Matthew's Gospel:
"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.
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."
In ecclesiastical heraldry, the keys symbolise the spiritual authority of the Papacy as Vicar of Christ, and by extension, priests. (See Who Holds the Keys?) In other words, the authority and responsibility to teach what is lawful or unlawful. Now as any educationalist knows, the success of a student depends not only on the teacher's ability to teach but also on that student's willingness to learn.
The same can be said for Christians. The Church can teach what is lawful or unlawful but each individual must fully understand and then be willing to accept what is taught.
Why two?
The number of keys in the Scriptures is not specified. The two keys in the papal insignia are in reference to the salvation, or the damnation, of our soul whilst we live this earthly life. If we are bound in the love of Jesus Christ on earth, then we will have our place in heaven. But if we allow ourselves to be loosened from Jesus Christ, then we will have no place in heaven.
And what does it mean to be "bound in the love of Jesus Christ"? Well, the two keys are arranged in the shape of a cross, and that gives us a clue. See Meaning of the Cross
https://www.ebay.com/itm/Pope-Pius-IX-Pectoral-Cross-All-Silver-Rare-chalice-co-/280340873904
Holy Mary, Queen (of Heaven)
See photo. Pope Pius added statues of Peter and Paul to St. Peter's Plaza.
Matt 16:18-19