In Homer’s Iliad at 8.13 we find the following, on the lips of Zeus:
Or seizing him I will hurl him into misty Tartarus, very far, where is the deepest gulf below earth; there are iron gates and brazen threshold, as far beneath Hades as sky is from earth.
Zeus speaks of the gates of Tartarus, far below Hades. The idea seems to be that there can be no escape from Tartarus – there is no way back to Hades, let alone the heavens (for it is other gods that Zeus threatens to cast down). Tartarus was part of a specifically Greek conception of the underworld. As an interesting aside, Tartarus is mentioned in 2 Peter 2:4, not as the destination for troublesome gods as in the Iliad, but as the destination for fallen angels. However, that must remain an aside.
Homer has Zeus speaking of gates in the underworld. We find the same theme in the words of Jesus, spoken to Peter:
And I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades shall not overpower it. Matthew 16:18
Jesus speaks of the ‘gates of Hades’. The expression is found in the Old Testament and in other Jewish literature (e.g. Job 38:17; Ps 9:13; 107:18; Is 38:10; Wis 16:13; 3 Macc 5:51; Ps Sol 16:2). However, in these cases the ‘gates of Hades’ is the threshold of the realm of the dead (associated with crossing that threshold in death, or in almost crossing it in a near-death experience). In the saying of Jesus in Matthew 16:18, we find the idea of gates overpowering or prevailing. Interpreters tend to find this puzzling, since in the ancient Near East city gates were a defensive, rather than an offensive, feature. However, in the Sibylline Oracles we find imagery that seems to reflect both Homer and the words of Jesus. The context is the resurrection of the dead:
But when the immortal God's eternal angels Arakiel, Ramiel, Uriel, Samiel, and Azael…will from dark gloom then lead to judgment all the souls of men before the judgment-seat of the great God…then the heavenly One give souls and spirit and voice, to them that dwell beneath and also bones fitted with joints unto all kinds of flesh, And both the flesh and sinews, veins and skin about the body, and hair as before; divinely fashioned and with breathing moved will bodies of those on earth one day be raised. Sib Or 2:214-226
The Oracle goes on,
And then will Uriel, mighty angel, break the bolts of stern and lasting adamant which, monstrous, hold the brazen gates of Hades, straight cast them down, and unto judgment lead all forms that have endured much suffering. Sib Or 2:227-230
In the Iliad, the gates of Tartarus prevent escape. In the Sibylline Oracles above, the gates of Hades are broken down in order to release the dead for judgement. The emphasis in the Oracle is clearly on the raising of the bodies of the dead. Those who are raised include Old Testament saints such as Moses, those killed in the flood, as well as Titans and giants (2.231-248)! Although the late date of the Sibylline Oracles places them after Jesus’s words, the emphasis on gates preventing the dead from being raised reflects the much earlier Homeric portrait in the Iliad of gates preventing the cast-down gods from returning to Olympus.
So, it seems to me that Matthew 16:18 is not really about evil powers frustrating the work of the church in the here and now (the most common interpretation of the verse), but about the victory of God’s people over death through resurrection. Jesus is building God’s new humanity, the church, as a people who will live human lives as human beings, body and soul. The church is a people destined for new life in a new earth. The reality of death will not defeat this purpose. Jesus, when he speaks these words to Peter, voices his mission: he has come to defeat death and to raise the dead – to save people body and soul. He rises from the dead to eternal life – and God’s people too will rise from the dead to eternal life. The fact that no-one has returned from Hades when Jesus speaks these words is about to be over-turned on the first Easter Sunday!