And Christ will say who is a christian and who is not. Not me, not you and not anyone else on this forum can judge who is a christian if they do not agree with that faith because of their own personal biases.
Actually, as Catholics, we do know which religion is and isn’t Christian, at least according to our definition. Everyone has their own definitions of what “Christian” means. According to some, everyone that believes in and worships Jesus Christ as God is a Christian. According to others, you must not only believe that, but you must believe in the traditional Christian concept of God. According to some definitions, Catholics aren’t Christians. According to others, Mormons aren’t Christians. Deal with it. We do. As Catholics, we believe that we are the original Christian Church, and we believe that anyone that departs from teaching on the nature of God isn’t a Christian. They may be a Christian according to the broad definition of anyone that believes in Jesus Christ and worships Him as God, but they aren’t Christian according to the strict definition of believing in God as formally defined at various Ecumenical Councils. Likewise, Catholics aren’t Christians according to the strict definitions of some. Again, deal with it. We do.
And obviously, that has nothing to do with whether someone is going to Hell or not, as the Catechism clearly states. Non-Christians can be saved by Jesus Christ. Academia would laugh if you told them that we can’t know who or what is Christian. Again, it depends on what definition you are using, since there are MANY that exclude various groups (including Catholics) depending on the definition.
And yes, ultimately God judges us. So as you tell us that God judges who is a Christian, you should also remember that God will judge whether someone that spent their life researching and critiquing mormonism was
wasting their life, in your words. One man’s garbage is another man’s treasure.

LOL.