You raise a good point, and you are halfway there, but you have stopped just short of what the text is actually revealing.
Yes, both “key” and “keys” denote authority.
But you assume that because both involve authority, the singular and plural forms are interchangeable.
They are not.
In Scripture, number always matters, especially when symbols are being used in prophetic, typological, or apocalyptic texts.
Let us start where you began: Isaiah 22:22.
“And I will place on his shoulder the key of the house of David; he shall open, and none shall shut; and he shall shut, and none shall open.” (Isa_22:22)
The Hebrew word here is מַפְתֵּחַ (maphteach), a singular noun from the root פָּתַח (pāthach, to open).
Eliakim is given the key, singular, because it denotes exclusive jurisdiction over the house.
This was a stewardship office, the Asher al-Habayith, second only to the king (cf. 1Ki_4:6, Isa_22:15), managing access to the royal chambers.
This is why the key is placed on his shoulder, symbolic of bearing authority (cf. Isa_9:6).
Now fast forward to Revelation 3:7:
“These are the words of the Holy One, the True One, who has the key of David, who opens and no one will shut, who shuts and no one opens.” (Rev_3:7)
The Greek here is τὴν κλεῖν Δαυείδ — ten klein Dauid — feminine singular accusative of κλείς (kleis, key).
It mirrors the Isaiah passage exactly because Christ is the fulfillment of that steward office, not as servant but as Son (Heb_3:6).
So yes, the singular key in Isaiah and Revelation points to exclusive messianic authority.
But here is where you missed the mark:
Matthew 16:19 does not say “key.” It says “keys,” plural, and that difference is the whole point.
“I will give you the keys of the kingdom of heaven…” (Mat_16:19)
Greek: δώσω σοι τὰς κλεῖς — future active indicative of δίδωμι (to give), plus τὰς κλεῖς, tas kleis — accusative feminine plural of κλείς (key).
It is plural, and not by accident.
So what gives?
Christ alone has the key of David (singular), meaning sovereign authority over salvation history.
But He delegates keys (plural) to Peter, which implies a representative stewardial authority rather than supreme sovereignty.
Just as Joseph had Pharaoh’s ring (Gen_41:42), but not Pharaoh’s throne.
Just as Eliakim had David’s key, but only as steward, while the king himself still ruled.
In Matthew 16, the plural keys reflect multiple dimensions of delegated authority:
Binding and loosing, legal-ecclesial categories rooted in rabbinic halakhah.
Doctrinal stewardship, a role later echoed in apostolic succession (cf. Act_15:7–11).
Judicial and evangelistic access, seen in Acts, where Peter opens the gospel first to Jews (Acts 2), then to Gentiles (Acts 10), in exact fulfillment of Christ’s commission.
Compare also Matthew 23:13:
“You shut the kingdom of heaven in people’s faces. For you neither enter yourselves nor allow those who would enter to go in.”
This is not metaphorical. The keys control access — proclamation, doctrine, judgment — not in arbitrary human fashion, but under the King’s decree.
So no, “key” and “keys” are not interchangeable.
They are layered.
The key of David, singular, speaks of the Messiah’s sovereign authority (Isa_22:22, Rev_3:7).
The keys of the kingdom, plural, speak of delegated apostolic authority to mediate, proclaim, and bind or loose within covenantal boundaries (Mat_16:19).
To collapse the plural into the singular is to flatten the typology and miss the distinction between ownership and stewardship.
Christ holds the key. No one else does.
Peter is given the keys, but only as servant in the house.
And finally, remember what Jesus said in Luke 11:52:
“Woe to you lawyers! For you have taken away the key of knowledge. You did not enter yourselves, and you hindered those who were entering.”
There again, key, singular, because true knowledge of God is one, and it belongs to Christ.
So do not blur the lines.
Let the singular remain singular.
Let the plural speak for itself.
There is one King.
And there are many servants.
The key opens the throne room.
The keys open the gates for others.
Both speak of authority.
But one is original, the other derivative.
One is Messianic.
The other is apostolic.
Shalom.
J.