For those of you who don't remember, the ancient Christians used to be ancient Pagans. Therefore, they cannot steal pagan customs that already belonged to them. Worship customs are created by the worshippers. They are not created by gods, they are created by people. Therefore, a custom cannot inherently idolatrous.
This is why memes like the one above are dumb as hell. They are usually perpetrated by people who are upset about wanting a Christmas tree but not particularly wanting Christ or usually Christians. It's okay, you can use it too, but we didn't STEAL the thing. It was ours to begin with. Much like if you're German or have German ancestry, it's your damn tree too. Use it. Cover it it in tinsel and stuff.