What does Luke 6:4 mean?
The Pharisees are criticizing Jesus' disciples for picking and eating grain as they walk through a field on the Sabbath. Jesus reminds them of when David, before he was king, asked for and received the bread of the Presence from the tabernacle. It's an odd comparison when read without sufficient background information.The Pharisees were a sect of Judaism that followed not only the Mosaic law but also the extra-scriptural oral law: regulations which scribes claimed God gave to Moses but weren't written down. In truth, the oral law was developed by scribes and rabbis who wanted to put a hedge around the Mosaic law so the people wouldn't break it and be sent into exile again (Jeremiah 1:14–16; 2 Kings 25:8–12).
After the fall of Jerusalem in AD 70, rabbis wrote out the oral law so the scattered Jews could have a standardized record. These are now known as the Talmud and Mishnah. The section of the Mishnah related to Sabbath rules is the Shabbat. One of the strongest sections on the Sabbath in the Bible (Exodus 31:12–17) directly follows the instructions for the tabernacle (Exodus 26:1—31:11). Scribes took this to mean that those who built and maintained the tabernacle still needed to follow the Sabbath.
This proximity led the scribes to align their rules about the Sabbath with the rules for the construction and maintenance of the tabernacle. The specific regulations they developed relate to things like tanning hides, as for the tabernacle cover; carrying things, like the pieces of the tabernacle; making and sewing fabric, like for the curtains that separate the Holy of Holies; and lighting and extinguishing a fire, like for the altar of incense.
The first part of the regulations, however, have to do with making bread. They include reaping and separating the husk from the kernel. This relates directly to the making of the bread of the Presence.
The disciples' act of plucking grain breaks the manmade rule related to the baking of the bread of the Presence. Yet, David actually ate the bread of the Presence (1 Samuel 21:1–6) which was reserved specifically and exclusively for the priests (Leviticus 24:5–9).
Jesus is indeed lord of the Sabbath (Luke 6:5), but He says something else Luke does not record: "The Sabbath was made for man, not man for the Sabbath" (Mark 2:27). All of it—the Sabbath, the bread of the Presence, the Mosaic law—was given to keep the Israelites in holy communion with God. For the Jews, it would, indeed, break the Sabbath to harvest grain, grind it, and make bread; that is work. And yet, the Sabbath is meant to be a blessing, not a burden, and if going without food is a burden, then it's okay to make the minimal effort to find food.