What will He do? He will sanctify us wholly. See the previous verse. He will carry on the work of purification till we are perfect in every part. He will preserve our "whole spirit, and soul, and body, blameless unto the coming of our LORD Jesus Christ."
Who will do this? The LORD who has called us out of darkness into His marvelous light, out of death in sin into eternal life in Christ Jesus. Only He can do this: such perfection and preservation can only come from the God of all grace.
Why will He do it? Because He is "faithful"--faithful to His own promise which is pledged to save the believer; faithful to His Son, whose reward it is that His people shall be presented to Him faultless, faithful to the work which He has commenced in us by our effectual calling.
It is not their own faithfulness but the LORD's own faithfulness on which the saints rely.
Come, my soul, here is a grand feast to begin a dull month with. There may be fogs without, but there should be sunshine within.