Gates of hell…those are strong picture words to use with the disciples. It means that no matter what comes up against His church there is nothing that can tear the fortress down! Notice also that Jesus didn’t say anything about ‘an order’ of priesthoods that we must follow.
“All men are liars who say they are of the true Church without the revelations of Jesus Christ and the Priesthood of Melchizedek, which is after the order of the Son of God.” Teachings of the Prophet Joseph Smith, pg. 375