English speakers think of a key as the thing that locks. Hebrew never did. Mafteach is built on the root patach, to open, and the noun cannot be said without the verb open sitting inside it. The object is named for the act it exists to perform. When the servants of Eglon finally take the mafteach and open the locked roof chamber in Judges 3:25, the word is doing exactly what it says.

The Tanakh hands the word its greatest weight in Isaiah 22:22, where the mafteach of the house of David is laid on a steward's shoulder: he shall open and none shall shut, he shall shut and none shall open. Revelation 3:7 lifts that line whole and puts it in Yeshua's mouth, with an open door no man can shut following in the next verse. The key of the house belongs to Him, and His stated use of it is opening.

The Greek of the Brit Chadashah keeps the same shape and then records the betrayal of it. In Luke 11:52 the word is kleis, key, and the charge is that the interpretive class took away the key of knowledge: did not enter, hindered the ones entering. In Matthew 23:13 the verb is kleio, to shut, from the same root family: the kingdom shut in people's faces. They were handed the opening thing and performed the shutting act with it. Usage governs and the root never proves doctrine, but the root does tell you how badly a tool was misused: they took an object named for opening and used it to lock.