Col 1:29 To this end I also labor, striving according to His working which works in me mightily
Phi 2:13 for it is God who works in you both to will and to do for His good pleasure.
Paul avows that he has power to “labor” and “strive” in the spirit for his converts only as Christ works in him and by Him (Gal 4:19). Paul refused to lean on self and chose to lean on the sufficient power of God to save and keep, this enabled him to strive because of God constantly working.
Paul accredits to God the Effectual calling (justification) and Effectual living (sanctification). They are all under the umbrella of the Effectual working of God without which all fails. God by His Spirit through His word and prayers make men effective in serving Him. That the willingness and the ability is not of us but of Him, just as in Him we live and move and have our being (Acts 17:28).