Isn't the "gates of hades" a term for death or the grave and that promise means that even death of a member will not mean the whole church has died?
Some also teach that the the city gate was where the elders met and "gates of hades" means the government or council or schemes of hades will not destroy God's Church.
One question about that second teaching. Israel was God's people before and even during the captivity in Babylon, weren't they? Did they cease to be God's people while in Babylon? Is it possible that the "Church" went into Babylon/Romish captivity for a while but still remained the Church and has since emerged from captivity? When Israel came out of Babylonian captivity they carried with them some "junk" they'd picked up there. Could the church today still carry some "junk" they picked up in captivity? Would carrying this "junk" cause them to cease being the church?
Some Romish/Babylonian "junk" in the church today could be:
1. The teaching (like in the notes of the Douay Bible at
John 3:5) that "born of water" means water baptism properly administered
2. Clergy as a ruling class to be obeyed unquestionably by the laity
3. Clergy holds salvation of the laity in their hands, you must stay in "the church" in order to receive eternal life through the sacraments
4. Large pyramid organization structure with one man (pope or GS) at the top and orders come down from headquarters to all
5. "Holy Tradition" like the manual has equal weight with written Word
6. Written Word only trustworthy as interpreted by clergy
7. Performance based religion or following of rules to get saved and to maintain salvation
8. Etc.