What does it mean to "cross yourself"? Alma 39:9

What does it mean to "cross yourself"? Alma 39:9

Alma tells his son Corianton to cross himself in the lusts of his eyes and his sins. 

Danny and I think that the more you think about this phrase, "cross yourself" the less it makes sense. When you just read it on a surface level, you can intuit what it means. It means to guard yourself, to put a barrier between you and x. But when you think about it more closely, it starts to sound strange.