Because I am so utterly sinful that I would turn back to sin and fall away from Christ if God did not keep me. I could never - and would never - do a single thing to please Him, except as He works in me to will and to do according to His good pleasure. Apart from Him, I can do nothing.
God, who elected us before the world began, gave us life, preserved us when we rebelled against Him, called us through the Gospel, regenerated us by the Holy Spirit, gave us faith, washed us in Christ's blood, and saved us by His grace, will certainly keep us to the very end and bring us safely into His glorious, eternal rest!
I think this is my all time favorite Keith Green song.