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"And the gates of hell … - Ancient cities were surrounded by walls. In the gates by which they were entered were the principal places for holding courts, transacting business, and deliberating on public matters...The word “gates,” therefore, is used for counsels, designs, machinations, evil purposes.

“Hell” means, here, the place of departed spirits, particularly evil spirits; and the meaning of the passage is, that all the plots, stratagems, and machinations of the enemies of the church would not be able to overcome it a promise that has been remarkably fulfilled." - Barnes Notes

http://www.studylight.org/commentaries/bnb/matthew-16.html

"The gates of hell,i.e, the machinations and powers of the invisible world. In ancient times the gates of fortified cities were used to hold councils in, and were usually places of great strength. Our Lord's expression means, that neither the plots, stratagems, nor strength of Satan and his angels, should ever so far prevail as to destroy the sacred truths in the above confession. Sometimes the gates are taken for the troops which issue out from them: we may firmly believe, that though hell should open her gates, and vomit out her devil and all his angels, to fight against Christ and his saints, ruin and discomfiture must be the consequence on their part; as the arm of the Omnipotent must prevail." - Adam Clarke Commentary

http://www.studylight.org/commentaries/acc/matthew-16.html