The Shia have been persecuted for over 1000 years, they didn't turn back on their faith. I dont know why you think is this special to early Christians who were a small group and existed underground to escape persecution until the ruler adopted their faith and imposed it.
you're indeed missing the point. it's pretty simple: if a religion is being fulfilled thru x, then one would reasonably expect this religion to persist and grow in numbers, rather than dwindle and disappear, thru x, and vice versa
if a religion is persisting and growing in numbers, rather than dwindling and disappearing, thru 'being persecuted and killed and without resisting persecution', then it is a reasonable indication that 'being persecuted and killed and without resisting persecution' is an essential part of this religion, that this religion is being fulfilled or is being true to itself, to its founder, thru 'being persecuted and killed and without resisting persecution'; case of Christianity (this is the point which you ought address)
likewise, if the shia sect persisted and grew in numbers thru 'being persecuted and killed while fighting and resisting against own persecution', then being 'persecuted and killed while fighting and resisting against own persecution' is an essential part of the shia sect (the sect's founder(s) e.g hussein). one would thus not expect their numbers to dwindle or their sect to disappear if/when they are being persecuted and killed while fighting against own persecution. however, their constant or continued fringe status within islam can be explained by them being predicated also upon mohammad where 'being persecuted and killed while fighting against own persecution' goes contrary to the nature of muhammad's message and the corresponding claims he spouted (proving all the more the point stated above and in my previous post)