V. 16.
Be always joyful;

The Message: Be cheerful, no matter what.
cf Nehemiah 8: 10 - 'Joy in the Lord is your strength'
NB: 'Joy' is not the same as 'happiness' as so many people think.  Happiness is a goal for many people: they want to be happy all the time.  This is not somehting human beings can achieve.  But we can be joyful, whether happy or  unhappy!

Dictionary definition of 'joy': a vivid emotion of pleasure, gladness, thing that causes delight.
This is not an emotion that we, as humans, can achieve all the time.  But, we can have pleasure and delight in a close relationship with God, no matter what our external circumstances.  We can delight in who God is, what he does, and what he has done for humankind, no matter what are our own circumstances.  Gladness and pleasure come because we know God accepts us (salvation) and we have hope for eternity.  We can have no doubts about that.

We need to let our joy be obvious.  This is one way we can be a witness for God.

V 17.
pray continually;

Pray all the time, not just in worship services or prayer meetings.  Pray regularly and frequently.
'Practise the presence of God'.  Talk with him about everything that's going on as if he's there beside us - our best friend (which he is!).  Don't forget to 'listen' for his replies - be aware of the many ways he does reply.  Have the 'antennas' up.

Rowland Croucher, in Grow! Meditations and Prayers for new Christians, p.124 talks about prayer.
  "How we pray is who we are.  Prayer is friendship with God  Our prayer is the best measure of the integrity of our Christian life...........
   Prayer covers all the events of our lives, so there are many different ways to pray...."
He does on to list many different ways we pray, both privately and together with other Christians.
   "Finally, a modern paraphrase of some advice from Julian of Norwich: Pray inwardly, even if you do not enjoy it.  It does good, even if you think you are doing nothing.  For when you are dry, empty, sick or weak, at such a time your prayer is most pleasing to God, though you find little enough to enjoy in it."

V 18
give thanks whatever happens; for this is what God in Christ wills for you.
We can give thanks to God in any circumstances, because God is there with us and we can delight in him no matter what.  Even when times are tough, God is still there, caring for us, loving us, and wanting the best for us.  He offers to strengthen us.  This is something to give thanks for.

Awareness of the things we have to be thankful for puts life in a proper perspective.  that's the way God wants us to live and behave.  When there is genuine joy, our witness is true and effective.

Vs. 19-22. A brief look at the Spirit and his gifts. (cf 1 Corinthians: 12-14)

V 19.
Do not stifle inspiration,
(NIV) Do not put out the Spirit's fire.
(The Message) Don't suppress the Spirit.
(TEV) Do not restrain the Holy Spirit
How could we do that?
(Quest Bible note, p. 1635) - by not acknowledging the work of teh Spirit and not giving him freedom to act in our lives and by trying to do everything in our own strength. By not using the gifts he has given to us to build up his church.

V 20
and do not despise prophetic utterances,
(NIV) do not treat prophesies with contempt
(The Message) don't stifle those who have a word from the Master.
(TEV)  do not despise inspired messages.

We must listen to other people - we need to live in community.  We can't be complete all on our own. God speaks to his people through each other.  No one person has all the knowledge of God's will.

Commentary p. 1284.  Gifts for ministry were being exercised, but some people were trying to suppress them, (we don't know just how) but it is wrong to do so.  In particular, Paul stressed the need to value the utterances of prophets.  Possibly the church had had a bad experience of them, and instead of exercising discernment between true and false prophesies was clamping dows on the whole activity.

V 21,  22.
but bring them all to the test and then keep what is good in them and avoid the bad of whatever kind.

A warning not to be gullible and accept everything we are told,
NOR to be skeptical and reject anything that seems different.
Check it out! Is it consistent with Scripture?  Is it consistent with Jesus' words and actions?  Is it endorsed by mature Christian leaders?

V 23.
May God himself, the God of peace, make you holy in every part, and keep you sound in spirit, soul and body, without fault when our Lord Jesus Christ comes.

God's work is making us what he wants/needs/intends us to be.  He is the source of our peace - spiritual blessings, of our becoming holy and our spiritual and physical health and of our salvation.  These are not due to anything we can do or say.

V 24.
He who calls you is to be trusted; he will do it.

God is completely trustworthy.  What he says, he will do.  He is worth our faith.