Many times we are told that God can either be this or that, but that He cannot be both. Same for our lives, really. In some instances, this is absolutely correct. So, God cannot be both good and evil. God cannot be both darkness and light. They are contradictions and God is absolutely non-contradictory. But sometimes, this and that do go together. One case, I’m going to suggest, is that He is totally Sovereign yet a respecter of your free will.
Paul writes in v.12 that we are to work out [our] own salvation with fear and trembling. He doesn’t mean that we work to earn it (that would contradict what he wrote elsewhere), but that we are to use this wonderful gift ourselves, activate it, put it to work in our lives. This takes decision and action in your life, doesn’t it, this takes you exercising your own free will to do so.
But, in v.13 he writes that this is to be done for it is God who works in you, both to will and to work for his good pleasure. Exercising our own free will is the what, and the why is here, because it is God who works in you. God works in you, so, use His Sovereignty and power and energies to work out your own salvation.
This might seem confusing; a Sovereign God works in me (v.13), and because of this I am to work as hard as I can to live a completely Christian life (v.12). David Guzik wrote this,
‘Those that are really God’s servants use their understanding of His sovereignty and omnipotence as a motivation for greater, more dedicated service to Him.’
God’s Sovereignty and our own free will are not as opposite as we might have been led to believe. They can and do work together. Simply, because we are serving a life giving, never ending, totally in control, ever flowing spring of omnipotence in a Sovereign God we are to take this and use this and put this to work in our own lives. As Paul writes here, it is God who works in you…so work out your own salvation!