Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
And David spake to the men that stood by him, saying, What shall be done
to the man that killeth this Philistine, and taketh away the reproach
from Israel? for who is this uncircumcised 
Philistine, that he should defy the armies of the living God?
And the people answered him after this manner, saying, So shall it be done to the man that killeth him.
And Eliab his eldest brother heard when he spake unto the men; and
Eliab's anger was kindled against David, and he said, Why camest thou
down hither? and with whom hast thou left those few sheep in the
wilderness? I know thy pride, and the naughtiness of thine heart; for
thou art come down that thou mightest see the battle.
And David said, What have I now done? Is there not a cause?
And he turned from him toward another, and spake after the same manner:
and the people answered him again after the former manner.
Vs.26-30
Two considerations, it seems, fired David with a holy indignation:--
(1.) That the challenger was one that was uncircumcised, a stranger 
to
God and out of covenant with him.
(2.) That the challenged were the
armies of the living God, devoted to him, employed by him and for him,
so that the affronts offered to them reflected upon the living God
himself, and that he could not bear.
How he was brow-beaten and discouraged by his eldest brother Eliab, who,
taking notice of his forwardness, fell into a passion upon it, and gave
David very abusive language.
Consider this,
(1.) As the fruit of Eliab's jealousy. He was the eldest
brother, and David the youngest, and perhaps it had been customary with
him to trample upon him and
take every occasion to chide him. Eliab intended, in what he said, not only to grieve and discourage David
himself, and quench that noble fire which he perceived glowing in his
breast, but to represent him to those about him as an idle proud lad,
not fit to be taken notice of.
(2.) As a trial of David's meekness, patience and constancy. A short
trial it was, and he approved himself well in it; for,
[1.] He bore the
provocation with admirable temper: "What have I now done? What fault have I committed, for which I should thus be chidden? He had right and reason on his side, and knew it, and
therefore did not render railing for railing, but with a soft answer
turned away his brother's wrath. This conquest of his own passion was in
some respects more honorable than his conquest of Goliath.
[2.] He
broke through the discouragement with admirable resolution. He would not
be driven off from his thoughts of engaging the Philistine by the
ill-will of his brother. Those that undertake great and public services
must not think it strange if they be discountenanced and opposed by
those from whom they had reason to expect support and assistance; but
must humbly go on with their work, in the face not only of their
enemies' threats, but of their friends' slights and suspicions.
Learn from David, too, to return quiet answers to those who would
roughly put you aside from your work.
Generally it is better to return
no answer at all.
David spake not so well by word as by deed.
His conduct was more eloquent than his language.
