Crossing oneself is an old tradition from the Catholic Church that was also taught in the beginning of the reformation to the ostracized Church who came to be known as Lutherans.
Martin Luther instructed all Christians to do so in his Catechism. Few besides Catholics do this today but it's just as appropriate for a protestant if they feel so inclined. I have many times, particularly when someone passes and I say a last prayer for their departed soul.
If you don't believe, I really don't care but that decision is on you. (Just crossed myself again for the non-believers here).
