Book of John Chapter 8
Just because I am a non believer does not mean I haven’t read parts of the Bible

5 Now Moses in the law commanded us, that such should be stoned: but what sayest thou?
6 This they said, tempting him, that they might have to accuse him. But Jesus stooped down, and with his finger wrote on the ground, as though he heard them not.
7 So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.
8 And again he stooped down, and wrote on the ground.
9 And they which heard it, being convicted by their own conscience, went out one by one, beginning at the eldest, even unto the last: and Jesus was left alone, and the woman standing in the midst.

Some people have interpreted writing in the sand as drawing a line in the sand, daring the scribes and Pharisees to cross it by throwing the first stone.

There is no real origin to the idiom; people have been drawing lines of demarcation since eh dawn of history. In addition to possible links to the story above from @josie, there is also “a Roman general who drew a circle in the sand around King Antiochus IV, ordering him not to cross it until he had replied to a Roman ultimatum.”

It certainly pre-dates cricket.

@josie my morning meditation today used that same passage to discuss the act of doodling while contemplating. The author (a Franciscan priest) suggested Jesus was doodling in the dirt.

I’m surprised no one has yet remarked on the elaborate rituals that are performed on naval vessels (and perhaps even on merchant ships, as well) for the first-time crossings of the Equator for the polliwogs among the crew who haven’t made that crossing yet.

But otherwise I’m in general agreement with those above: drawing lines on the ground (or drawing lines on representations of the ground, i.e., maps) has been going on for who knows how long. (Certainly since before maps were invented.) And “crossing those lines” with various stated or implied intent has nearly always carried consequences of one kind or another.

“Crossing the line” is no more idiom than “coming over here” or “standing over there”; it’s clear on its face.

https://youtu.be/C6hmQwfEmzc?t=1m10s