Ping!
People really need to read the whole context of the passage:
Php 2:13 for it is God who works in you, both to will and to work for his good pleasure.
Php 2:14 Do all things without grumbling or disputing,
Php 2:15 that you may be blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine as lights in the world,
Php 2:16 holding fast to the word of life, so that in the day of Christ I may be proud that I did not run in vain or labor in vain.
It is God who works in us. God is the true engine for all things. And in verse 13 Paul tells us that 1) God enacts His will in us, and 2) God works through us for His pleasure. We become His servants or slaves as Romans states. This is exact the total OPPOSITE of losing one's salvation.
As noted in verse 14 and part of 15. Once we become Christians we are to submit to God's will-not ours. We are to rejoice in all things because it is the will of God for us and He is working through us. We trust Him because this is His promise to us-that we will bear good fruit. And that is a promise to all Christians-whether we can see that fruit or not. We are lights shining in darkness. So, in good times and in bad, whether we get that promotion or not, we rejoice because God means it for our good though we might not understand why.
When we grumble or complain, we bring disrepute on God. We simply show that we don't trust Him. We don't trust His will for our lives and we don't trust that He's working through us. This is reflected in our behavior and shows a poor testimony to the world. Our light to the world is diminished.
The whole letter of Philippians is about rejoicing and being glad in the mist of struggles and adversity.
This is a very difficult lesson for Christians-probably the most difficult. And, considering God has called us and is working very hard through us, we should be fearful and tremble simply so that we don't bring shame to His holy name. Not because we fear we'll lose our salvation. But simply because of our love for the Father.