Make the story itself carry the message. Do not, at any point, tell your readers how they should interpret it. If they figure it out on their own, it will be much more powerful to them. Even if they don't figure it out consciously, they'll probably absorb it by osmosis.
Every story has a message of some sort, even if the author hasn't consciously included it. Your worldview will colour everything you write. What you think is important, or right, or wrong, or meaningless, is going to show through somewhere, no matter what you do. The shallowest story about shoes and shopping is delivering the message that these things matter, because it's asked you to spend several hours of your life living in that world.
If you think environmentalists are heroes, if you write a book about them, chances are they are going to come off pretty well, despite any flaws they might have. If you think they are dangerous delusionals, that's going to show also, even if that idea is never made explicit in the dialogue or the narrative. If you have a more nuanced opinion of them, that also will show, in the characterization, in the outcome, in the language, in the methods. You will never have to tell the reader what you think of environmentalists, not even by proxy in the mouth of another character.
So don't make the message explicit and it will go over much better.