Glad hearts are a tonic for everybody.  Nobody really wants to be surrounded by grumpy, morose, complaining, anxious or angry people.  Yet it is easy enough to become like that ourselves.  All we need is a little trouble of any kind, and we can spiral down into self-preservation mode, denying our security in Christ.  Paul is in prison, but he instructs 'free' people to rejoice in the Lord!  He does not apologise for repeating himself (Philippians 1:18; Philippians 2:17-18; Philippians 4:4; Philippians 4:10), because he knows that rejoicing is an essential spiritual protection.

Nehemiah, whom God appointed to rebuild the walls of Jerusalem, spurred on his workforce at a time when they were worn out and overawed by God’s Word, by telling them that “… the joy of the Lord is your strength” (Nehemiah 8:10).  The determination to turn away from their weakness and failure and look to the Lord in joyful thanksgiving would give them new strength to dedicate themselves to the Lord for the future.   

Joy protects our hearts but only if our joy is 'in the Lord'.  'In the Lord' describes the reason for being glad: it is because of our Father's love, our Saviour's sacrifice and the Holy Spirit's work in our hearts (Philippians 4:4).  As we fix our attention on God, and His care of us, joy will start to flood our hearts, displacing fear and shame.  Naturally we will seek assurance based on what we can see in our circumstances.  Supernaturally we can be certain that God will keep His promises and work powerfully by His Spirit.

Paul experienced this himself when he and Silas were in jail in Philippi (Acts 16:25-26).  Humanly-speaking the prospect was not good.  But they sang praise to the God who is in charge of everything, and were not surprised when the doors burst open and they were free.  Praise expresses the joyfulness of confident hearts, which are certain that God will be kind and active.  So, why not sing God's praise?  Sing and speak out gladly about His love ... on your own, in the family, with the children, when travelling and at work.  If you have a workplace fellowship, please practise rejoicing together - especially if things are not going well!!  You will be surprised what happens when you declare your confidence in the unseen God.