It is God who saves! God saved Israel from Egyptian slavery. Israel did not overcome Pharaoh and Egypt by either warfare or by dint of superior intelligence. Jesus Christ is our Savior, and we cannot save ourselves from sin's power. When we accept Him as Savior, it obligates us as His servants to obey Him.
In like manner, when God broke Egypt's power, enabling the Israelites to be free, it obligated them to walk out of Egypt if they wanted their liberty. If the Israelites wanted to save their skins and be totally free of Egypt when God parted the Red Sea, it obligated them to walk the path God made for them between the walls of water. When God said, "I am going to bring you into the land and provide for you along the way," if Israel wanted these things, they were obligated to walk all the way to Canaan. It is very clear that if Israel wanted what God said He would give, then Israel had to also cooperate by working in the form of walking to where God said He would lead or take them. "Walk" is a code word for living.
With this as a background, when Paul says to "work out your own salvation," it cannot possibly mean we are going to save ourselves. Rather, like what confronted the Israelites when God opened the way to their physical salvation from slavery in Egypt, we should be ready to make God's spiritual salvation practical and operational. Paul does not say we must work for salvation, but rather carry our salvation out to its conclusion. He uses "work out" in much the same sense as when a student is told to work out an arithmetic problem—to bring it to its conclusion. For us, the conclusion, the goal, to work toward is Christ-likeness. The salvation here is sanctification, victory over sin unto holiness.
To make it very plain, if we want to be one with Him, we must get moving in the direction He is pointing, and He points toward His standards of conduct and attitude. Each person's walk is not exactly the same because each person's experience and makeup are somewhat different. There is enough similarity among humans, though, to make the Bible always relevant.
One of the beautiful things about this is that each person's walk toward the image of God is exactly right for him. What is more, Philippians 2:13 also says God gives us both the will and power or energy to do it! The New Testament in Modern Speech renders it, "For it is God Himself whose power creates within you both the desire and the power to execute His gracious will." This work of God in us is another aspect of His grace, and without it, we could never be one with Him.
God Himself produces in us both the desire to live righteously and the effective energy to do so. He does not demand what we cannot do (I Corinthians 10:13). We see in Philippians 2:12 our responsibility and in verse 13 help to accomplish it.
We can see this working together with God in simple illustrations from physical life. We may launch a sailboat upon the water, but it takes what God supplies, wind, to make it move. We may plant vegetable seeds, but it is the power of God in nature that makes the plant grow and produce food. We may generate gigawatts of electricity in power plants, but God provides the wind, water, sunlight, coal, oil, or gas to turn the turbines. In each case, we add something to what God already supplied.
Our salvation is something already given because it is God's will, and He is sovereign. We, though, must do something to make it practical by applying ourselves to salvation's demands. Even in this, God enables us to do it!
We will never know where the dividing line is between what God supplies and what we are responsible to do because it is different for each according to God's purpose. This proportion must be different because each person is different, and He is preparing us for different responsibilities within His Family. This is sure, however: Our walk toward salvation will always be difficult enough to be challenging and edifying.
Even here the analogy of Israel in the wilderness comes to our aid. Where did Israel get the energy to walk across the wilderness to the Promised Land? Did it not come from the manna and water God supplied, as well as the vision and hope of the inheritance which He also provided? Still, Israel walked! They had to work, to cooperate.
The verb "work" in Philippians 2:12 is in a tense that indicates continuous working. Just as Israel did not leave Egypt and arrive in the Promised Land in one step, neither are our salvation and oneness with God accomplished at once. It, too, is a process; it is our life's work.