A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
1 Kings (1 Samuel) 21:1
sa1 21:1DAVID, AT NOB, OBTAINS OF AHIMELECH HALLOWED BREAD. (Sa1 21:1-7)
Then came David to Nob to Ahimelech--Nob, a city of the priests (Sa1 22:19), was in the neighborhood of Jerusalem, on the Mount of Olives--a little north of the top, and on the northeast of the city. It is computed to have been about five miles distant from Gibeah. Ahimelech, the same as Ahiah, or perhaps his brother, both being sons of Ahitub (compare Sa1 14:3, with Sa1 22:4-11, Sa1 22:20). His object in fleeing to this place was partly for the supply of his necessities, and partly for comfort and counsel, in the prospect of leaving the kingdom.
Ahimelech was afraid at the meeting of David--suspecting some extraordinary occurrence by his appearing so suddenly, and in such a style, for his attendants were left at a little distance.
1 Kings (1 Samuel) 21:2
sa1 21:2The king hath commanded me a business, and hath said unto me, Let no man know--This was a direct falsehood, extorted through fear. David probably supposed, like many other persons, that a lie is quite excusable which is told for the sole purpose of saving the speaker's life. But what is essentially sinful, can never, from circumstances, change its immoral character; and David had to repent of this vice of lying (Psa 119:29).
1 Kings (1 Samuel) 21:4
sa1 21:4there is hallowed bread--There would be plenty of bread in his house; but there was no time to wait for it. "The hallowed bread" was the old shew-bread, which had been removed the previous day, and which was reserved for the use of the priests alone (Lev 24:9). Before entertaining the idea that this bread could be lawfully given to David and his men, the high priest seems to have consulted the oracle (Sa1 22:10) as to the course to be followed in this emergency. A dispensation to use the hallowed bread was specially granted by God Himself.
1 Kings (1 Samuel) 21:5
sa1 21:5these three days--as required by law (Exo 19:15). David and his attendants seem to have been lurking in some of the adjoining caves, to elude pursuit, and to have been, consequently, reduced to great extremities of hunger.
the bread is in a manner common--that is, now that it is no longer standing on the Lord's table. It is eaten by the priests, and may also, in our circumstances, be eaten by us.
yea, though it were sanctified this day in the vessel--that is, though the hallowed bread had been but newly placed on the vessel, the ritual ordinance would have to yield to the great law of necessity and mercy (see on Mat 12:3; also see Mar 2:25; Luk 6:3).
1 Kings (1 Samuel) 21:6
sa1 21:6there was no bread there--in the tabernacle. The removal of the old and the substitution of the new bread was done on the Sabbath (Lev 24:8), the loaves being kept warm in an oven heated the previous day.
1 Kings (1 Samuel) 21:7
sa1 21:7Doeg, an Edomite--who had embraced the Hebrew religion.
detained before the Lord--at the tabernacle, perhaps, in the performance of a vow, or from its being the Sabbath, which rendered it unlawful for him to prosecute his journey.
the chiefest of the herdmen that belonged to Saul--Eastern monarchs anciently had large possessions in flocks and herds; and the office of the chief shepherd was an important one.
1 Kings (1 Samuel) 21:9
sa1 21:9HE TAKES GOLIATH'S SWORD. (Sa1 21:9)
sword of Goliath--(See on Sa1 17:54).
behind the ephod--in the place allowed for keeping the sacred vestments, of which the ephod is mentioned as the chief. The giant's sword was deposited in that safe custody as a memorial of the divine goodness in delivering Israel.
There is none like that--not only for its size and superior temper, but for its being a pledge of the divine favor to him, and a constant stimulus to his faith.
1 Kings (1 Samuel) 21:10
sa1 21:10AT GATH HE FEIGNS HIMSELF MAD. (Sa1 21:10-15)
David . . . fled . . . to Achish the king of Gath--which was one of the five principalities of the Philistines. In this place his person must have been known, and to venture into that country, he their greatest enemy, and with the sword of Goliath in his hand, would seem to have been a perilous experiment; but, doubtless, the protection he received implies that he had been directed by the divine oracle. Achish was generous (Sa1 27:6). He might wish to weaken the resources of Saul, and it was common in ancient times for great men to be harbored by neighboring princes.
1 Kings (1 Samuel) 21:13
sa1 21:13feigned himself mad--It is supposed to have been an attack of epilepsy, real or perhaps only pretended. This disease is relieved by foaming at the mouth.
let his spittle fall down upon his beard--No wonder that Achish supposed him insane, as such an indignity, whether done by another, or one's self, to the beard, is considered in the East an intolerable insult.