John Gill Commentary 1 Samuel 12:21

"and turn ye not aside; for [then would ye go] after vain things which cannot profit nor deliver, for they are vain." — 1 Samuel 12:21 (ASV)

And turn you not aside
From his worship: for then; if they turned aside from that:

should you go after vain things ;
idols, which are vanity, and less than vanity:

which cannot profit nor deliver ;
neither bestow good things on their votaries, nor deliver them from evils, or from the hands of their enemies

for they are vain ;
empty, useless, and unprofitable; an idol is nothing in the world, (1 Corinthians 8:4) .