Here are some ancient keys, a door knob (center), and the remains of the mechanism (upper right) which would have probably be nailed to a large wooden door.
Courtesy of Allard Pierson Museum
These keys are typical of ones found during the period of the Roman Empire which is the Early Christian Era. However, keys were known even earlier in the Kingdom of Judah, as reflected in Isaiah 22.22 where Eliakim son of Hilkiah is prophesied to be given the key to the House of David. This key, Isaiah tells us, gives power so that “what he opens no one can shut, and what he shuts no one can open.” Jesus is the One whom this prophecy ultimately refers, as seen in Rev. 3.7-8 where Jesus promises the church at Philadelphia to use this key, giving them the same benefit. So some keys, in the N.T., only Jesus carries. Another key that belongs exclusively to Jesus, is the key to death and Hades. In Rev. 1.18, Jesus describes Himself as the “Living One; I was dead, and now look, I am alive for ever and ever.” He then claims to possesses “the keys to death and Hades,” which, presumably, shows the ability to grant everlasting life to humans and control of the nether regions to carry away those belonging to Him. Also, the ability to keep imprisoned those who are not His seems implicit. Unquestionably, the The Eternal Son always possessed immortality, but the incarnated Son of Man, who took upon himself our humanness, needed to be given eternal life (“I was dead”) as the second Adam. At least this is how I understand the human aspect of the second Adam. Henceforth, he is able to grant this same eternal life to His followers whom He represents.
These keys, belonging to Jesus, seem to be different from the “keys of the kingdom of heaven” given to Peter and the other disciples in Mt. 16.19 (and probably Mt. 18.18). Only in Mt. 16 are they referred to as “the keys of the kingdom of heaven” without explanation of what exactly is being bound and loosed. In Mt. 18, the retention of sins to the unrepentant in the Christian community is the thing bound. This is instruction about church discipline from Jesus. Ultimate intransigence would lead to a break in fellowship (vs. 17). An intermediate step in the process would be denial of the table of bread and wine since this occurred every Lord’s Day. It would prevent further judgement upon the offender, since, the observance carries inherent risk of punishment when partaken improperly (see 1 Cor. 11.27-32). Loosing of sins is always available to those who ask for forgiveness (vss. 21-22). based upon my understanding, the keys in Mt. 16 and 18 are identical.
Jesus seems to reiterate this teaching when He appears to His disciples on the evening of resurrection day saying they all possessed the power to either forgive or retain sins (Jn. 20.23). Here, this seems to be part of an evangelistic function when people truly accept the message of Christ. Perhaps this is part of the “all authority in heaven and on earth” spoken about in Mt. 28.18. No longer would worshippers need to formally bring a sacrifice to the priests in Jerusalem to have their sins forgiven. The High Priesthood of Jesus is inaugurated and the typological sacrifices at the central sanctuary have now been fulfilled.