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See notes below from MacArthur Study Bible

2:12 obeyed. Their faithful response to the divine commands Paul had taught them (cf. Ro 1:5; 15:18; 2Co 10:5, 6). work out your salvation. The Gr. verb rendered “work out” means “to continually work to bring something to fulfillment or completion.” It cannot refer to salvation by works (cf. Ro 3:21–24; Eph 2:8, 9), but it does refer to the believer’s responsibility for active pursuit of obedience in the process of sanctification (see notes on 3:13, 14; Ro 6:19; cf. 1Co 9:24–27; 15:58; 2Co 7:1; Gal 6:7–9; Eph 4:1; Col 3:1–17; Heb 6:10, 11; 12:1, 2; 2Pe 1:5–11). fear and trembling. The attitude with which Christians are to pursue their sanctification. It involves a healthy fear of offending God and a righteous awe and respect for Him (cf. Pr 1:7; 9:10; Is 66:1, 2). † 2:13 God who is at work in you. Although the believer is responsible to work (v. 12), the Lord actually produces the good works and spiritual fruit in the lives of believers (Jn 15:5; 1Co 12:6). This is accomplished because He works through us by His indwelling Spirit (Ac 1:8; 1Co 3:16, 17; 6:19, 20; cf. Gal 3:3). to will and to work. God energizes both the believer’s desires and his actions. The Gr. word for “will” indicates that He is not focusing on mere desires or whimsical emotions but on the studied intent to fulfill a planned purpose. God’s power makes His church willing to live godly lives (cf. Ps 110:3). good pleasure. God wants Christians to do what satisfies Him. Cf. Eph 1:5, 9; 2Th 1:11.