with fear and trembling--the very feeling enjoined on "servants," as to what ought to accompany their "obedience" ( Eph 6:5 ). So here: See that, as "servants" to God, after the example of Christ, ye be so "with the fear and trembling" which becomes servants; not slavish fear, but trembling anxiety not to fall short of the goal ( 1Cr 9:26, 27 Hbr 4:1 , "Let us fear, lest a promise being left us of entering into His rest, any should come short of it"), resulting from a sense of our human insufficiency, and from the consciousness that all depends on the power of God, "who worketh both to will and to do" ( Rom 11:20 ). "Paul, though joyous, writes seriously" [J. J. WOLF].
Jamieson, Fausett & Brown
"Servant" implies work. "Work" is the first word in the verse. St. Paul puts it in the imperative. He mentions fear, an emotion meaningless if the salvation had been already assured. Why do you twist the scripture like that? The meaning is clear, we must work out our salvation every day. The next verse, about God driving the work is not there to negate the previous verse, but to explain that grace is necessary for all good work.