Verse 4. Rejoice in the Lord alway. See Barnes "Php 3:1".

It is the privilege of Christians to do this, not at certain periods and at distant intervals, but at all times they may rejoice that there is a God and Saviour; they may rejoice ill the character, law, and government of God -- in his promises, and in communion with him. The Christian, therefore, may be, and should be, always a happy man. If everything else changes, yet the Lord does not change; if the sources of all other joy are dried up, yet this is not; and there is not a moment of a Christian's life in which he may not find joy in the character, law, and promises of God.

{a} "Rejoice in the Lord alway" Re 22:7,20