Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; 13 for it is God who works in you both to will and to do for His good pleasure. Philippians 2:12-13 NKJV 

The apostle Paul instructed the Philippians to give their salvation a workout. To workout your salvation is to obey and produce fruit of your faith. God saves by grace through faith (Eph. 2:8), and faith without works is dead—i.e., without fruit (Jas. 2:17). God not only saves by grace, He also gives you energy (spiritual power) to workout your salvation to produce fruit, and the source of that energy is a supernatural supply of the Spirit (Phil. 1:19).

A famous football coach (George Allen) said that, “a workout is a key that helps unlock opportunity and success. Hidden within each of us is an extraordinary force.” Biblically speaking, that force is the Holy Spirit (Acts 1:8). The Spirit of God is a force (active power) that energizes, influences, and strengthens, exerting power and bringing it to bear in the varied circumstances and situations in life (see Merriam-Webster Dictionary).

The Holy Spirit is a force that operates in the region of the “heart.” Man is a spirit and he possesses a soul that lives in a body (1 Thess. 5:23).  The word “heart” is used figuratively throughout the Scriptures for identifying the spirit of man—the place within where all thoughts and feelings are generated. There can be no salvation in Christ Jesus without an active power of the Spirit (Rom. 10:9-10, 13).

The force of the Spirit needs to be understood before and after the resurrection: There was always a force of the Spirit, even the in Old Testament, that God used to move upon His servants “both to will and to do for His good pleasure” (text). However, after Jesus was glorified via His resurrection, the force of Spirit now operates like “rivers of living water” inside the believer (John 7:37-39), manifesting in a variety of ways (1 Cor. 12:3-6). The Holy Spirit is a force of energy that energizes you to workout your own salvation. Just work it out!