This is one of the most ludicrous WT teachings, and that's saying a lot. This particular interpretation of prophecy assumes that the UN actually has authority to do such a thing. THEY DON'T! The UN cannot enforce its own decrees. They have no military. To worst they can do is sanction a country that violates its orders. Can anyone imagine the UN issuing a decree banning religion and people waking up in Saudi Arabia the next day tearing down their mosques in response? Of course not.
I really struggled with this one when I was an active Witness. I had to go through some arduous mental gymnastics to make it work in my mind. I would tell myself that, even though it doesn't make sense, Jehovah says that he will put this thought into the minds of the world leaders, and Jehovah can make anything possible. Why, people scoffed at Noah when he told them God would bring a deluge and their laughter didn't stop until the doors of the ark were closed...
Deep down inside, I was just waiting for 'new light" to clean that mess up. The possibility of a world body issuing a decree banning religion is less of a possibility now than it was at any point in the last century. Back when the Soviet Union was the great enemy of the U.S., you could potentially envision a scenario where the Soviet Union would win the Cold War and promulgate atheism in order to further their cause, but they've long disappeared from the planet. Many experts believe that the world is in the middle of another Great Awakening. Religion and religious fanaticism in particular, is growing throughout the world, not just in the Middle East, but in Latin America and the United States as well. Religion isn't going anywhere, folks, no matter what the U.N. has to say about it.