True Christian joy is not the same as happiness. Happiness, rooted in the idea of “hap,” or chance, is a pleasant feeling triggered by good circumstances, health, success, or possessions, and it passes as easily as those circumstances change. Joy, by contrast, is a settled sense of well-being that can be experienced even in the middle of difficulties, pain, sickness, and the uncertainties of life, because it does not depend on circumstances at all. Paul can say the kingdom of God is not eating and drinking but righteousness, peace, and joy in the Holy Spirit, and the psalmist says fullness of joy is found in God’s own presence.
This joy is a supernatural response to believing the gospel; it is grounded not in subjective feelings but in the objective, solid truth of what God has done for us in Christ, his victory and reconciling work at the cross. That is why Peter can describe believers rejoicing with joy inexpressible even while grieved by various trials, since their faith’s proof is being tested like gold refined by fire. It is also why Jesus tells his disciples not to rejoice merely that spirits are subject to them, but that their names are written in heaven, the greatest joy of every Christian.