If someone is trusting in works for salvation, then one could never have assurance of salvation because they could never know if they did enough good works. God's standard is perfection so its a fail right out of the starting gate (Romans 3:23) and there's only one way we will stand before God as righteous. (Romans 4:5-6; Philippians 3:9) We certainly are saved FOR good works (Ephesians 2:10) but not by good works. (Ephesians 2:8,9)
In regard to Philippines 2:12, notice that Paul said to "work out" your salvation and NOT "work for" your ssalvation.This is in regard to sanctification. When we "work out" at the gym, we exercise to develop our body that we already have and not to get a body. Farmers "work out" the land, not in order to get the land, but to develop the land they already have. The Greek verb rendered "work out" means "to continually work to bring something to completion or fruition." We do this by actively pursuing the process of ongoing sanctification, which is the result of being set apart for God's work and involves the process of being conformed to the image of Christ.
This conforming to Christ involves the work of the person, but it is still God working in the believer to produce growth and maturity in the person who has already been justified by faith. In verse 13, Paul goes on to say, "for it is God who works in you both to will and to do for His good pleasure." Ongoing sanctification has no bearing on justification. That is, even if we don't live a sinless, without fault or defect, flawless, absolute perfect life 100% of the time (which we won't) we are still justified by faith in Christ. (
Romans 5:1) Where justification is a legal declaration that is instantaneous, ongoing sanctification is a process.
In regard to "fear and trembling," it pertains to a healthy fear of offending God through disobedience and a righteous awe and respect or reverence for Him. (
Proverbs 1:7;
Psalm 2:11;
Psalm 34:9;
Isaiah 66:2) Not fear of God as if He is an unloving tyrant. Paul uses the same phrase "fear and trembling" in
2 Corinthians 7:15 in which he refers to Titus as being encouraged by the reception of him by the Corinthians "with fear and trembling," that is, with humility and respect for his position as a minister of Jesus Christ. In
1 Corinthians 2:3, we see that Paul himself came to the Corinthian church in "weakness and fear, and with much trembling" in regard to the huge responsibility and critical importance of the work in which he was engaged.
Assurance of salvation is found only in Jesus Christ. 1 John 5:11 - And this is the testimony: that God has given us eternal life, and this life is in His Son. 12 He who has the Son has life; he who does not have the Son of God does not have life. 13 These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.