New International Version (©1984) And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it.
Note he uses the word Hades, not Hell. Most Christians know less about Heaven and Hell than the average Greek pagan of Jesus' time. Look up Hades to learn what Jesus was really talking about.
The entire sentence in Greek is really awesome. The "gates of Hades" will not get the upper hand, that is death cannot cheat the redemption offered by Christ Jesus.
Jesus knew the score and picked his words accordingly. It's an honest mistake to make, though, when you're relying on the precepts of men. They obscure the truth, but revelation by revelation the truth gets out.
Stick with the Bible and what it says and you cannot go wrong.
You may want to tell your mormon friends that...
Apparently, they didn't get the memo.