Jesus said of his church, that the gates of hell will not stand against it (Mt. 16:18). A high profile and controversial book recently said that the gates of hell are a defensive stance, that is, the gates stand defensively, so we followers of Christ need not be concerned about them. That is to say, the gates of hell are stationary and cannot come after us, and so, not to worry. However, in Greek culture and language, gates were seen as synonymous with rulers, or those in power (see Vines), so could easily have been a reference to the powers of hell, which will not prevail against the church in the end. As such, they need not be defensive at all, but may in fact be offensive in strategy. A similar figure of speech today might be, for example "pillars of the faith" in reference to leaders and scholars.
In any case, one has to take scripture as a whole, not in part, before arriving at such conclusions. To that end, please consider the following passages written to Christians:
1 Peter 5:8 "Be self controlled and alert. Your enemy the devil prowls around like a roaring lion looking for someone to devour."
1 Tim. 3:7 "He (an overseer) must have a good reputation with outsiders so that he will not fall into disgrace and the devil's trap."
1 Tim. 5:15 "Some have in fact already turned away to follow Satan."
2 Cor 2:11 "In order that Satan might not outwit us. For we are not unaware of his schemes."
Ephesians 6:12 "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places."
Beloved, we owe it to ourselves, and to future generations, to search the scriptures to see whether what we are being taught is accurate (Acts 17:11). Many things sound right at first blush, and so error is introduced to a generation, and each generation can drift a few inches farther from truth. God forbid! Let each generation represented take seriously their responsibility to guard both the grace and the truth of our Lord Jesus Christ. Only then can we, the church, prevail against the very gates of hell.