In 1 Thessalonians 5:16-18, the apostle Paul lists three things that we should do as believers. First, we should rejoice. The Greek word translated here as rejoice is chairōs, which means to be filled with joy. As followers of Christ, we have every reason to be filled with joy. Because Jesus' sacrifice for us, we have been released from the bondage of sin, we have access to the presence of God, and we have eternal life. Paul says we should rejoice always. So, we should be joyful, not just when we feel like it, but always. Keep in mind that joy and happiness are two different things. Happiness is based on circumstances, where joy is not. No matter what is going on in our lives, good or bad, we should always be filled with joy because of what Christ has done for us. The word chairōs is also used in Matthew 18:13 to illustrate the joy that God experiences in the reconciliation of His children to Him through Christ. If that makes God rejoice, we should also rejoice.
The second thing that Paul says we should do is to pray. But he doesn't say we should pray once a week, once a month, or once in a blue moon. He says we should pray continually or without ceasing. This doesn't mean that we should be reciting some formal prayers 24/7. What it means is that we should look to talk to God everyday of our lives and as often as we can. Prayer should be part of our everyday lives. After all, prayer is conversation with God and we should want to be in conversation with Him frequently. When our hearts and our minds are truly focused on God, then this will come naturally.
The third thing Paul tells us we should do is to give thanks. The Greek word that translates as give thanks is eucharisteō, which also means to be thankful. In the context in which it is used, it specifically means giving thanks to God. This is the same word that is used when Jesus gave thanks to God at the Passover meal on the night before He died for us (Matthew 26:27; Mark 14:23; Luke 22:17). We have much to be thankful to God for. Without Him, nothing would exist. We owe everything to Him. We should be thankful for the incredible love that God has shown us by allowing His Son, Jesus, to pay the penalty for our sin. Paul says that we are to give thanks in all circumstances. We should be thankful to God in happiness and in sorrow, in trials and in victories, in sickness and in health. No matter what circumstances we find ourselves in, the love that God showed in giving His only Son for us will never fade. And that is something we can always be thankful for.
Paul ends these verses by saying that we should do all these things--rejoicing, praying, and giving thanks--for one very important reason. It is God's will for us in Christ Jesus. God's desire is for us to filled with joy, to talk to Him every day, and to be grateful, not just for all that He has done, but also for His love. God is our heavenly Father and, as His children, it should be our desire to be obedient to Him by doing His will.