2:12-13 What does Paul mean when he says "work out your own salvation with fear and trembling"?

Paul is exhorting Christians here to make the self-abnegation and humility displayed by Jesus in Php 2:5-8 a fact in their own lives (cp Php 2:5-8). The words work out mean to "carry out to its ultimate conclusion."

This does not mean that salvation is by works, but that Christians are individually responsible for obedience to God's word in the process of sanctification (cp Mt 6:19-20; Jn 6:27; 2Cor 13:5; 1Ti 6:17-19; 2Pe 1:10; Jude 1-3).

Fear and trembling in Php 2:12 refers to the attitude with which Christians are to pursue their sanctification. It involves a healthy fear of offending God and a dread of sinning against Him, and of the consequences that may follow (cp Psa 2:11-12; 15:1-5; 85:9; 119:119-120; Pr 3:7-8, 9:10; 14:27; 16:6; Ecc 12:13; Isa 66:1-2; Lk 12:4-5; 1Cor 9:26-27; 10:12-14; Eph 6:5-8; He 6:4-6; 10:26-31).

While believers are responsible for working out their salvation, it is God who actually produces the good works and spiritual fruit in their lives (cp Php 2:13 with Hos 14:8; Jn 15:5; 1Cor 12:6; 2Cor 3:5; 1Ti 1:12).

It must be clearly understood though that God can only produce good works and fruit in those who conform to His word and allow the Holy Spirit to outwork in them. "Once saved" does not mean "always saved", as some teach (cp Mt 3:10; 7:19-27; Lk 13:6-9; Jn 15:1-8; Col 1:22-23).