Paul was "of the world" until he wasn't. So there you have it, God *can* save the whole world.
That's nations. *And* the goats are separated because of their lack of *works* (i.e. feeding hungry, clothing poor) during the tribulation that was imminent. So unless you're a works-salvation person, this should make you rethink what Jesus is talking about here. It's not about the final judgment at Jesus' physical return.
The narrow gate is the way of peace, which is Jesus' way. Many chose to go the way of violence and were swept away in "the flood" at the coming destruction rather than heeding Jesus' words to flee when the armies were surrounding Jerusalem.
One can't create an entire doctrine on a parable. Jesus *never* mentions hell. Hell is mentioned *only one time* in scripture and it's referring the abode of fallen angels ("Tartarus") awaiting judgment (2 Peter 2:4). So, hell is not eternal and it's not for humans.
I know many who adhere to the doctrine and they are some of the most committed Christians I have ever met.
Maybe it's your doctrine that's a lie?
Romans 5:18
Therefore, as one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men.
If all means all in the first half then all means all in the second half.