The Passover Episode

Moses has two sons at this point, but there's only one here. So in order to protect him, and anybody from getting killed is to circumcise him. The firstborn of Israel is not going to die by this plague of death because of the blood of the lamb. Now it's still weird. There's probably really good reasons why this is here. But at the end of the day, you know, couldn't God have simply told Moses all this earlier?