Only conduct yourselves in a manner worthy of the gospel of Christ: monon axios tou euaggeliou tou Christou politeuesthe (2PPMM): (Phil 3:18-21; Eph 4:1; Col 1:10; 1Th 2:11 2:12; 4:1; Titus 2:10; 2Pe 1:4 1:5 1:6 1:7 1:8 1:9 ; 3:11 3:14 ) (See Torrey's Topic "Christian Conduct") (Ro 1:9; 1:16 15:16;15:29 2Co 4:4; 9:13; Gal 1:7) (Spurgeon's devotional)

Whatever happens, conduct yourselves in a manner worthy of the gospel of Christ (Lightfoot 1)

But under all circumstances do your duty as good citizens of a heavenly kingdom; act worthily of the Gospel of Christ (Lightfoot 2)

One thing you must see to whatever happens—live a life that is worthy of a citizen of the Kingdom and of the gospel of Christ. (Barclay) 

But whatever happens, make sure that your everyday life is worthy of the Gospel of Christ (Phillips)

Only (since my only reason for remaining on earth is for your progress in the Christian life), see to it that you recognize your responsibility as citizens (of heaven), and put yourselves to the absolute necessity of performing the duties devolving upon you in that position, doing this in a manner which is befitting to the gospel of Christ (Wuest)

Only (3440) (monon) conveys the idea of the one essential thing. It focuses attention on the next few words of Paul.

Conduct yourselves is a command (imperative mood) in the present tense, calling for continuous conduct. The middle voice means the Philippians must initiate the action of proper conduct themselves and participate in the results thereof (or stated another way the subjects -Philippians saints- act in reference to themselves and for their own benefit, participating in the results of the action.)

Conduct (4176) (politeuomai from polítes = citizen from pólis = city) means to conduct one's self according to the laws and customs of the state (cf note on Philippians 3:20).  Philippi as a colony possessed Roman citizenship (see note below) and Paul was proud of his own possession of this right, but even "prouder" of his heavenly citizenship. And so the Philippians who also are citizens of heaven should behave accordingly. They should be in practice what they are in position, which applies to saints of all times.

Just as Philippi was a colony of Rome with all the rights and privileges bestowed on those who were born in Rome, in a similar way the church at Philippi was a colony of heaven, and the members were commanded to walk as citizens of that unseen country, not so much by keeping outward regulations, but by the power of the indwelling Holy Spirit. Their worthy walk was to reflect the transformation wrought in them by the gospel of Jesus Christ. God wanted the Philippians' daily conduct to betray them as pilgrims and strangers on earth & as citizens of  "the city which has foundations, whose architect and builder is God" (see note Hebrews 11:10), citizens of "a better country, that is, a heavenly one" (see note Hebrews 11:16) and of "the city of the living God, the heavenly Jerusalem and to myriads of angels" (see note Hebrews 12:22).  A life worthy of the gospel is a life lived in the power of the Holy Spirit. It is the earthly walk of a heavenly minded man.

William Barclay phrases it this way

Philippi was a Roman colony; and Roman colonies were little bits of Rome planted throughout the world, where the citizens never forgot that they were Romans, spoke the Latin language, wore the Latin dress, called their magistrates by the Latin names, however far they might be from Rome. So what Paul is saying is, “You and I know full well the privileges and the responsibilities of being a Roman citizen. You know full well how even in Philippi, so many miles from Rome, you must still live and act as a Roman does. Well then, remember that you have an even higher duty than that. Wherever you are you must live as befits a citizen of the Kingdom of God. (Barclay, W: The Daily Study Bible Series. The Westminster Press or Logos)

This phrase is placed at beginning of Gk sentence for emphasis -- the idea is

only worthily of the gospel of Christ conduct yourselves

A manner worthy is one Greek word (axios) (Click for study of axios) which means having the same weight as another thing & can refer to a set of scales that balance with the same weight on one side as on the other side of the scale. So if Christ is in me (one side of the scale), then my goal in this body of flesh is to walk worthy, making those decisions that are in accordance with Who is in me (the other side of the scale). The idea is that the conduct of the saints weigh as much as the character of Christ The Philippian saints (and all saints) were to see to it that their manner of life weighs as much as the gospel they profess to believe. That which gives weight to a Christian’s words is the fact that his manner of life befits, is congruous to and corresponds with the gospel he preaches. If your conduct doesn't match your profession you may not have a genuine possession (of the gospel of salvation).

Walking in a manner worthy (click here for Scriptural summary of a "worthy walk") is a phrase used several times in the NT (see notes Ephesians 4:1; 1Thessalonians 2:12, 3Jn 1:6) with the use in Colossians 1:9-12 (Click Col 1:9, 1:10 for exposition) giving an excellent synopsis of what a "worthy walk" should look like:
 

Illustration - The world famous master of mime, Marcel Marceau, was asked what the difference was between regular acting and pantomime. Marceau's response was interesting. He said, "In the case of a bad actor, the words are there even if the actor is no good. But when a mime is not good, there is nothing left. A mime must be very clear and very strong.  The same thing is true of the Christian's" conduct and witness. Is it "conduct which is worthy of the gospel?". If a believer's verbal testimony is rejected, it may be wise for him to say no more. But it's then that silence should speak so clearly that no one can mistake the message. If we are in a situation where our actions alone have to do the talking, let's make sure they are coming through loud and clear for Christ. When our lives honor Christ, even silence is eloquent.

The gospel of Christ are the good tidings of the Kingdom of God and of salvation through Christ, to be received by faith, on the basis of His expiatory death, His burial, resurrection, and ascension. (See commentary notes on the gospel as defined in 1Corinthians 15:1; 15:2; 15:3; 15:4; 15:5; 15:6; 15:7; 15:8)

Warren Wiersbe tells the following story illustrating a worthy walk...

We have some neighbors who believe a false gospel,” a church member told his pastor.

Do you have some literature I can give them?

The pastor opened his Bible to 2Corinthians 3:2,

You are our letter, written in our hearts, known and read by all men

He said,

The best literature in the world is no substitute for your own life. Let them see Christ in your behavior and this will open up opportunities to share Christ’s Gospel with them.

The greatest weapon against the devil is a godly life. And a local church that practices the truth, that “behaves what it believes,” is going to defeat the enemy. This is the first essential for victory in this battle. (Wiersbe, W: Bible Exposition Commentary. 1989. Victor or Logos)

Spurgeon (Morning and Evening) writes...

The word "conversation" (conduct) does not merely mean our talk and converse with one another, but the whole course of our life and behaviour in the world. The Greek word signifies the actions and the privileges of citizenship: and thus we are commanded to let our actions, as citizens of the New Jerusalem, be such as becometh the gospel of Christ. What sort of conversation is this? In the first place, the gospel is very simple. So Christians should be simple and plain in their habits. There should be about our manner, our speech, our dress, our whole behaviour, that simplicity which is the very soul of beauty. The gospel is pre-eminently true, it is gold without dross; and the Christian's life will be lustreless and valueless without the jewel of truth. The gospel is a very fearless gospel, it boldly proclaims the truth, whether men like it or not: we must be equally faithful and unflinching. But the gospel is also very gentle. Mark this spirit in its Founder: "a bruised reed he will not break." Some professors are sharper than a thorn-hedge; such men are not like Jesus. Let us seek to win others by the gentleness of our words and acts. The gospel is very loving. It is the message of the God of love to a lost and fallen race. Christ's last command to his disciples was, "Love one another." O for more real, hearty union and love to all the saints; for more tender compassion towards the souls of the worst and vilest of men! We must not forget that the gospel of Christ is holy. It never excuses sin: it pardons it, but only through an atonement. If our life is to resemble the gospel, we must shun, not merely the grosser vices, but everything that would hinder our perfect conformity to Christ. For his sake, for our own sakes, and for the sakes of others, we must strive day by day to let our conversation be more in accordance with his gospel.

So that whether I come and see you or remain absent I will hear of you that you are standing firm in one spirit: hina eite elthon (AAPMSN) kai idon (AAPMSN) humas eite apon (PAPMSN) ta peri humon akouo (1SPAS) hoti stekete (2PPAI) en eni pneumati: (Phil 2:12 2:24) (Eph 1:15; Col 1:4; 1Th 3:6; Phile 1:5; 3Jn 1:3 1:4) (Eph 2:1 2:2; 4:1; Ps 122:3; 133:1; Mt 12:25; 1Cor 1:10; 15:58; 16:13 16:14; 2Cor 13:11)

Then, whether I come and see you or only hear about you in my absence, I will know that you stand firm in one spirit (Lightfoot 1)

So that whether I come among you and see with my own eyes, or stay away and obtain tidings from others, I may learn that you maintain your ground bravely and resolutely, acting by one inspiration (Lightfoot 2) 

in order that whether having come and having seen you, or whether being absent I am hearing the things concerning you, namely, that you are standing firm in one spirit holding your ground (Wuest)

so that whether I come and see you, or whether I go away and hear how things go with you, the news will be that you are standing fast (Barclay) 

So that whether I do come and see you, or merely hear about you from a distance, I may know that you are standing fast in a united spirit (Phillips),

Standing firm (steko) conveys the ideas of firmness or uprightness & means in essence “to stand firm and hold one’s ground."  "Steko" indicates the determination of a soldier who stands his ground not budging one inch from his post. Like "good soldiers of Christ Jesus" (see notes 2 Timothy 2:3; 2 Timothy 2:4) they were to stand fast as a united front, the body of Christ. The idea is to keep one’s ground in battle, and in order to do this, one must be settled on the firm foundation of the Rock of Ages. Paul's exhorts spiritual solidarity and an esprit de corps. The Holy Spirit unites Christians into one body (1Cor 12:13, see note Ephesians 4:4). If they can stand firm in the Spirit, they can overcome small differences among individual members and work forcefully toward a common goal—to withstand external persecution related to the gospel. In one spirit means in the spirit of unity and harmony; for bickering, contention, and self-seeking hinder and mar the gospel witness.

Withstanding The Storm (Our Daily Bread) - When we stand together as Christians, we can endure the fierce onslaughts of adversity and the subtle attacks of our spiritual foe. But when we isolate ourselves, or if we're abandoned by those who should be there to uphold us, we are more likely to fall. This is illustrated by the towering fir trees of Washington state. A few years ago a violent windstorm swept through the area and many trees that were standing by themselves were blown over. But those that grew tightly together in the forest did not fall because they were strengthened by their intertwining roots. They were able to withstand the strong winds. What they couldn't do by themselves, they were able to do together. As followers of Jesus, we need each other. The apostle Paul told the believers in Philippi to "stand fast in one spirit" and to help one another as they faced adversity (Phil. 1:27; 2:3-4). We too are to join arms and hearts, offering encouragement and hope in our struggles. We may not know when a fellow Christian is being tossed around by the strong winds of adversity. But we do know this: When we stand together, we're better able to withstand the storm. --D C Egner (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

We're stronger when we stand together

with one mind striving together for the faith of the gospel