Jamieson, Fausset and Brown on 2 Peter 1:8

8. be--Greek, "subsist" that is, supposing these things to have an actual subsistence in you; "be" would express the mere matter-of-fact being (Ac 16:20).

abound--more than in others; so the Greek.

make--"render," "constitute you," habitually, by the very fact of possessing these graces.

barren--"inactive," and, as a field lying fallow and unworked (Greek), so barren and useless.

unfruitful in--rather, ... in respect to, "The full knowledge (Greek) of Christ" is the goal towards which all these graces tend. As their subsisting in us constitutes us not barren or idle, so their abounding in us constitutes us not unfruitful in respect to it. It is through doing His will, and so becoming like Him, that we grow in knowing Him (Joh 7:17).