Because grace is extended to you on behalf of Christ, not only to believe in him but also to suffer on his behalf - Philippians 1:29
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We are saved by grace, grace has been extended to us, to believe but also to suffer on behalf of Christ. In Acts we read Paul went about, "strengthening the souls of the disciples, encouraging them to continue in the faith, and saying that through many tribulations we must enter the kingdom of God" (Acts 14:22). If you have read the epistles you know that Paul suffered much, and he experienced abandonment from men during such times and comfort from others. Paul was able to strengthen the brethren because he had been strengthened and he was able to comfort and encourage because he had received comfort while suffering greatly for Christ. Truly, there is understanding in this when he writes, "rejoice with those who rejoice; mourn with those who mourn" (Romans 12:15). People who know this truth in Christ, having suffered with him, will mourn with those who are suffering, giving comfort because they have received comfort. Christ exists as our great priest who can sympathize with our weakness because he too suffered in the flesh. There is purpose in suffering, it conforms us to Christ, ridding us of the old self, bringing us into matruity, in greater knowledge and revelation of Christ, so we can share with others.
whether now distressed, on behalf of your encouragement and salvation, whether encouraged on behalf of your encouragement, that working in endurance, of the same sufferings which we also suffer - 2 Corinthians 1:6
Paul writes that our Father of mercies and God of comfort encourages us in all our afflictions so that we may have the power to encourage the one in affliction through the encouragement we ourselves are encouraged by God, "because just as the sufferings of Christ abound toward us in the same way also the encouragement through Christ overflows" (2 Corinthians 1:3-5). So we see what it means to have fellowship in the sufferings of Christ. It draws us closer to him, giving us greater revelations of knowing him, thus Paul could say, "in my flesh, I am filling up what is lacking in Christ's afflictions for the sake of his body, that is the church." Christ suffered for the Church. Paul suffered for the church. Through suffering many sons are brought to glory.
... but that with full courage now as always Christ will be honored in my body, whether by life or by death. For to me to live is Christ, and to die is gain. If I am to live in the flesh, that means fruitful labor for me... to remain in the flesh is more necessary on your account... for your progress and joy in the faith, so that in me you may have ample cause to boast in Christ Jesus - Philippians 1:20-26
To remain in the flesh meant fruitful labor for him, in regards to progress and growth in the church, but it meant continued sufferings for Paul. Paul writes he faced death all day. "For your sake, we face death all day, considered like as sheep to be slaughtered" (Romans 8:36). How could he face suffering and death's door every day? "In all these things we are winning a most glorious victory through the one loving us" (Romans 8:37). Paul was deeply rooted and founded in the love of Christ. "Who can separate from the love of Christ" when we experience "all things things" tribulations, distress, persecution, famine, nakedness, danger, and sword. Notice, he writes famine and nakedness. Does not Jesus say he will provide for our needs and we are not to worry about food nor clothing? Paul experienced many dangers, he was shipwrecked, thrown in prison, beaten with rods, flogged, stoned and left for dead, thrown in to fight wild animals... and he writes he went hungry and thirsty and naked many times (2 Corinthians 11:23-27).
I face death every day, brothers, as surely as I boast about you in Christ Jesus our Lord. If I fought wild beasts in Ephesus for human motives, what did I gain? - 1 Corinthians 15:32-33
How could Paul endure? "To this end I labor, striving according to the power that works in me" (Colossians 1:29). What was Paul striving for, to what end did he labor and face such hardships? "Now I rejoice in my sufferings for your sake, and in my flesh, I am filling up what is lacking in Christ's afflictions on behalf of his body, that is, the church" (Colossians 1:24). Why was he suffering on behalf of the church? "the one we proclaim, persuading all men, teaching all men in all wisdom so that presenting all men mature in Christ" (Colossians 1:24). This is the very thing he says God has purposed in the church, so the body grows up into mature adulthood (Ephesians 4:11-15). Though joy is found in the trial and peace within the storm, it takes the storms and trials to conform us to Christ, to learn these things.
that I know him and the power of his resurrection and the fellowship of his sufferings, conforming to his death if somehow to attain the resurrection from the dead - Philippians 3:10-11
Conforming to his death, having fellowship in his sufferings, so to experience his life. The teachings of baptism tell us "for if we have been united with Him like this in His death, we will certainly also be united with Him in His resurrection" (Romans 6:5). Paul wrote, " accordingly, daily I die, by the boasting in you which having in Christ Jesus our Lord" (1 Corinthians 15:31). Paul died daily for their progress of faith and joy. Jesus said, "if any man will come after me, let him deny himself, and take up his cross, and follow me" (Matthew 16:24). "Truly, truly, I say to you, unless a grain of wheat falls into the earth and dies, it remains alone; but if it dies, it bears much fruit" (John 12:24). Peter writes, whoever has suffered in the flesh has ceased from sin (1 Peter 4:1-11). Fellowship in Christ, in suffering, has a purpose; to put to death the desires of the flesh. Thus the greatest sacrifice we can give God is to present our bodies a living sacrifice unto holiness (Romans 12:1).
for I now am already poured out like a drink offering and the time of my departure is near, the good fight fought, the race finished, the faith kept - 2 Timothy 4:6-7
Paul writes suffering manifests the life of Christ, as we are being conformed to his death, life is brought to this mortal body. "For we who live are constantly being handed over to death for the sake of Jesus, so that the life of Jesus also may be manifested in our mortal flesh" (2 Corinthians 4:11). This is why Paul strived and endured such hardships, it was for the progress and joy of the church. And John tells us our joy comes through Christ as we have fellowship in him as the Word of Life, Eternal life (1 John 1:1-4). Therefore, suffering conforms us to His death, so that we might experience life in Christ.