Said this law to the individual soul, "Work out your salvation with fear and trembling, for it is God that worketh in you to will and to do of His own good pleasure." To will and to do.

It is His own good will and pleasure that the soul shall be fitted and lifted up to its high destiny through this co-working.

It was His power to raise it to that condition without man's participation or conscious acquiescence; but it was His will and pleasure to enact this law of salvation.
Looking across the circumference of the individual soul, what says this law?
"Go ye out into all the world and preach the gospel to every creature, and, lo, I am with you unto the end,"-- not as an invisible companion, not merely with the still, small voice of the Comforter to cheer you in trial, weakness and privation; but with you as a _co-worker_, with the irresistible energies of the Spirit of Power.

He might have done the whole work alone.

He might have sent forth twelve, and twelve times twelve legions of angels, and given each a voice as loud as his who is to wake the dead, and bid them preach His gospel in the ears of every human being.