The gates of hell shall not prevail

Then I saw an angel coming down from heaven, holding in his hand the key to the bottomless pitand a great chain. 2 And he seized the dragon, that ancient serpent, who is the devil and Satan, and bound him for a thousand years, 3 and threw him into the pit, and shut it and sealed it over him, so that he might not deceive the nations any longer, until the thousand years were ended. After that he must be released for a little while.7 And when the thousand years are ended, Satan will be released from his prison 8 and will come out to deceive the nations that are at the four corners of the earth, Gog and Magog, to gather them for battle; their number is like the sand of the sea. 9 And they marched up over the broad plain of the earth and surrounded the camp of the saints and the beloved city, but fire came down from heaven and consumed them, 

What does the binding of Satan stand in contrast to? How does Satan unbound behave compared to Satan bound? Well, in one respect, Satan bound can't "deceive the nations." But what does that amount to?

According to 20:7-8, to "deceive the nations" means to recruit an army against "the saints." His deceptive ability enables him to rally the troops. That's consistent with premillennialism. 

But it's also consistent with amillennialism. Notice that v8 depicts the assault on a global scale. A worldwide military campaign. 

During the church age, Satan is bound in the sense that he's unable to mount a concerted attack on global Christianity. He can wage and win regional battles. But although the church is dying in some localities, it is spreading to other localities. And even persecution can backfire. An underground church movement may be bigger, more vigorous than fair-weather churches. He loses by winning. Satan can attack the Christian community at different times and places, but he can't snuff it out.