David Ate The Showbread
When Saul sought his life, David fled. His first stop is in the city of the priests, Nob, and David was greeted by the high priest, Ahimelech. It was at this time the priest gave David and his men the holy bread and sped them on their journey.
The bread of the Presence is clearly restricted to the priests inside the tent.
Leviticus 24:5-9
You shall take fine flour and bake twelve loaves from it; two tenths of an ephah shall be in each loaf. And you shall set them in two piles, six in a pile, on the table of pure gold before the LORD. And you shall put pure frankincense on each pile, that it may go with the bread as a memorial portion as a food offering to the LORD. Every Sabbath day Aaron shall arrange it before the LORD regularly; it is from the people of Israel as a covenant forever. And it shall be for Aaron and his sons, and they shall eat it in a holy place, since it is for him a most holy portion out of the LORD's food offerings, a perpetual due.
Nonetheless, Ahimelech decided to forego his meal to give it to David and his men.
1 Samuel 21:3-6
"Now then, what do you have on hand? Give me five loaves of bread, or whatever is here." And the priest answered David, "I have no common bread on hand, but there is holy bread--if the young men have kept themselves from women." And David answered the priest, "Truly women have been kept from us as always when I go on an expedition. The vessels of the young men are holy even when it is an ordinary journey. How much more today will their vessels be holy?" So the priest gave him the holy bread, for there was no bread there but the bread of the Presence, which is removed from before the LORD, to be replaced by hot bread on the day it is taken away.
What David and Ahimelech did was wrong even though they viewed it as support for God's chosen king. Jesus is not excusing this behavior.
Jesus mentions David here to draw attention to the larger picture: God's king is among the people, but someone else is ruling and seeking his life.
Ahimelech knew David was God's chosen king, and he clearly supported him. Where is the clear support due to Jesus?
"Guiltless"
The disciples of Jesus are doing nothing wrong by plucking and eating grain on a Sabbath. Jesus said they were "guiltless" (Matthew 12:7).
The Law supports this assertion.
Deuteronomy 23:24-25
If you go into your neighbor's vineyard, you may eat your fill of grapes, as many as you wish, but you shall not put any in your bag. If you go into your neighbor's standing grain, you may pluck the ears with your hand, but you shall not put a sickle to your neighbor's standing grain.
Leviticus 19:9-10
When you reap the harvest of your land, you shall not reap your field right up to its edge, neither shall you gather the gleanings after your harvest. And you shall not strip your vineyard bare, neither shall you gather the fallen grapes of your vineyard. You shall leave them for the poor and for the sojourner: I am the LORD your God.
Are the poor not allowed to eat on the Sabbath? And why are Jesus and His disciples poor?
"If David did it, so can we!"
People sometimes think Jesus is claiming the right to break the rules like David did.
However, the disciples are not in fact breaking the Law by plucking grain to eat. (See "Guiltless" above.) So, we know the Lord is not excusing wrongdoing.
There is a serious problem with thinking Jesus is using David's behavior to excuse His disciples: This interpretation leads to situational ethics.
If the circumstances call for it, this reasoning goes, you can bend the rules a little to fit the situation--especially when you are an authority figure. This mistaken thinking is all too common in human institutions.
On the contrary, the Bible consistently reminds us to take our dedication to God with the utmost sincerity and all seriousness.
Deuteronomy 23:21-23
If you make a vow to the LORD your God, you shall not delay fulfilling it, for the LORD your God will surely require it of you, and you will be guilty of sin. But if you refrain from vowing, you will not be guilty of sin. You shall be careful to do what has passed your lips, for you have voluntarily vowed to the LORD your God what you have promised with your mouth.