The best way to keep cats out of the Christmas tree is to put it in a place where they can't reach it. If the tree is up too high, the cat may not be able to jump up there. If the tree is on the floor, try putting something heavy in front of it, like a sofa or table, so the cat can't easily push it over. You can also try using a Christmas tree collar, which is a circular piece of fabric that wraps around the base of the tree and prevents the cat from climbing up it.