12 Wherefore, my beloved, as ye 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 which worketh in you both to will and to do of his good pleasure.

This thought began in the first chapter where Paul pointed out that work and suffering in this world goes with being a Christian, but the result is worth it. chapter 1:12, "But I would that ye should understand, brethren, that the things which happened unto me have fallen out rather unto the furtherance of the gospel," and, verses 29-30, "For unto you it is given in the behalf of Christ, not only to believe in him, but also to suffer for his sake, having the same conflict which ye saw in me, and now hear to be in me."
In chapter 2 Paul says, verse 5, "Let this mind be in you, which was also in Christ Jesus," and goes on to show that Christ willingly left the glory of heaven and came to earth to suffer and die for our sins. WHEREFORE, God hath highly exalted him. WHEREFORE, my beloved, having the same mind-set as Christ, be willing to work out (exercize, exhibit) your salvation, taking this trust very seriously, because God is directing you to do so.

14 Do all things without murmurings and disputings,
15 That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world;
16 Holding forth the word of life; that I may rejoice in the day of Christ, that I have not run in vain, neither laboured in vain.

Remember, you are the sun and moon, guiding lights, to a wicked and perverse nation. Make certain that your behavior is that of a child of God.

Paul may be may alluding to those towers which were built at the entrance of harbors, on which fires were kept during the night to direct ships into the port. Genuine Christians, by their holy lives and conversation, are the means of directing others, not only how to escape those dangers to which they are exposed in human life, but also of leading them to eternal safety and rest.

[That I have not run in vain] When I am with Christ, I will rejoice to see that my work and suffering were productive.

17 Yea, and if I be offered upon the sacrifice and service of your faith, I joy, and rejoice with you all.
18 For the same cause also do ye joy, and rejoice with me.

The price of such productivity may be his life. Paul, considering their faith and testimony to be an offering to God, states that his martyrdom, if it should occur, would be the libation (drink offering), i.e. wine poured upon a sacrifice. Should I be thus offered I shall rejoice, and you should also rejoice, that I am counted worthy of this high honor.

19 But I trust in the Lord Jesus to send Timotheus shortly unto you, that I also may be of good comfort, when I know your state.

[But I trust in the Lord Jesus] He is the governor and director of all events, being above all in principality and power; and I humbly hope that I shall be spared a little longer so that I may send Timothy to you.

[When I know your state[ By the correct information which I shall receive from Timothy.

20 For I have no man likeminded, who will naturally care for your state.
21 For all seek their own, not the things which are Jesus Christ's.

[For I have no man likeminded] Epaphroditus, his companion in labor and messenger from Philippi, was on his way home because he had been ill and the Philippians were concerned. There was none other, save the youthful Timothy, who shared Paul's zeal and affectionate concern for the Philippians.

[For all seek their own] Their own agenda was more important to them than doing that which was most important to Christ. See chapter 1:15 and chapter 2:3-4.

22 But ye know the proof of him, that, as a son with the father, he hath served with me in the gospel.

[Ye know the proof of him] The Philippians were well acquainted with the affectionate attachment of Timothy to Paul, for he had labored with him there, as we learn from Acts 16:1-3 and Acts 17:14.

23 Him therefore I hope to send presently, so soon as I shall see how it will go with me.

[How it will go with me] The apostle was now in captivity; his trial appeared to be approaching and he was doubtful of the result; though he seems to have had a general persuasion that he should be spared.

24 But I trust in the Lord that I also myself shall come shortly.