A little context helps. The song was written and released while the Vietnam War was still going on, and Richard Nixon was president. Anti-war demonstrations were taking place all over the world. Meanwhile Lennon had found it difficult to enter the US because of an earlier drug conviction. So he was living in Toronto, sort of in exile. This was also before his song Imagine, but very much on the same theme. War is over (if you want it.). Imagine there's no war, its easy if you try. So it's that kind of juxtaposition of subjects that might be a bit on the sad side. His life was a bit of a struggle at the time, and he was just hoping to convince people to have the faith that all the negativity could be overcome with positive thoughts. What makes it popular and so enduring after almost 50 years? Probably the fact that it was sung by a Beatle, and at that time, any of the Beatles could release anything and it would go to #1.