EWTN Catholic Q&A
CCC paragraph 676
Question from Daniel on 06-16-2017:

We had some bigshot priest connected to the Vatican that goes around preaching tree-planting. He kept talking about how "we" will make a beautiful world, etc. It gave me the creeps, because he seemed to think that man was going to solve whatever's wrong and not God. Is this an example of what they're talking about in 676? The wording is a little complicated for me, can you please put the paragraph in simpler terms so I can recognize these things when I see and hear them? Thank you

Answer by David Gregson on 12-18-2017:

What the Catechism calls "millenarianism" is any doctrine that teaches or presumes that any social or political program can bring about an age of perfect peace. Only Christ can do that, and He won't do it until His Second Coming, when He returns to judge the living and the dead.

Whether the priest you mentioned is guilty of millenarianism would depend on what he hopes for from any human program. If it's just a matter of cleaning up the environment, picking up trash and planting flowers, he can't be faulted for that. If he expects it to overcome evil, then obviously he is misguided.