“Rejoice in the Lord always; again I will say, rejoice” (Phil 4:4) is the exhortation of Saint Paul for the Christians of Philippi to remind them that their “commonwealth is in heaven” (3:20) and that they must lead a life “worthy of the gospel of Christ” (1:27), “in humility” not looking only to their own interests, but also to the interest of others. (cf. 2:3-4). The Apostle writes about joy while he is in chains, and those who will receive his letter have adversaries, are suffering and fighting the same battle as Paul (Cf. 1:28-30) and need watch out for the Judaizers (Cf. 3:2-3).
For Christians, joy is not the result of a easy life without difficulties or a change of circumstances or mood, but a profound and constant attitude that is born of faith in Christ: “We know and believe the love God has for us.” (1 John 4:16). The goal of the Christian message transmitted to us is communion with God so that “our joy may be complete” (1 John 1:4).
God desires that all men are happy, He created us for eternal life, starting on earth through grace and reaching its consummation in heaven where man is united with God forever: “Although man can forget God or reject Him, He never ceases to call every man to seek him, so as to find life and joy”. That’s why the message of the Gospel is an invitation to all men to enter in this joy that is communion with Christ: “The joy of the Gospel fills the hearts and lives of all who encounter Jesus. Those who accept his offer of salvation are set free from sin, sorrow, inner emptiness and loneliness. With Christ, joy is constantly born anew”. Indeed, the Gospels describe many encounters with Christ, source of our happiness: John the Baptist leaped from joy in Saint Elizabeth’s womb when he felt the presence of the Incarnated Word (Cf. Luke 1:41); the shepherds received the announcement of “good news of a great joy which will come to all the people; for to you is born this day in the city of David a Saviour, who is Christ the Lord”(Luke 2:10-11); When the Wise Men saw the star that lead to the King of Jews “they rejoiced exceedingly with great joy” (Matt 2:10); the joy of the paralytic, blind, leprous and all others that were cured by Jesus; the joy of the widow of Nain to see her son resurrected.(Cf. Luke 7:15-16); the joy of Zacchaeus overflows into a banquet and a deep conversion (Cf. Luke 19:6-8); the joy of the Good Thief, in the middle of his physical sufferings in the cross, knowing that he would be with Jesus in His Kingdom on the same day (Cf. Luke 23:42-43); and finally the joy of Mary Magdalene, the disciples of Emmaus and the Apostles when they saw Christ Resurrected. The only encounter of the rich young man with Jesus does not end with joy, because he did not use his freedom to follow the Master: “but when he heard this he became sad, for he was very rich”. (Luke 18:23).
Nature of Joy
Joy is a passion produced by an encounter with something that we love, a feeling or sensation of pleasure that is not only sensible but is accompanied by rationality. Saint Thomas Aquinas explains in his treatment of the passions in the Summa Theologica: “For we take delight both in those things which we desire naturally, when we get them, and in those things which we desire as a result of reason. But we do not speak of joy except when delight follows reason; and so we do not ascribe joy to irrational animals, but only delight”. When asked if joy was a virtue, Saint Thomas answers that joy doesn’t appear in theological, moral or intellectual virtues, “hence joy is not a virtue distinct from charity, but an act, or effect, of charity: for which reason it is numbered among the Fruits… (Gal 5:22)”. Indeed, Christian joy is a consequence of possessing God by Faith and Charity, the fruit of living all the virtues.
Joy is one of the fruits of the action of the Holy Spirit in the soul, that consists substantially in identifying ourselves with Christ and calling God Abba, Father: “For all who are led by the Spirit of God are children of God” (Rom 8:14). “Recognizing this utter dependence with respect to the Creator is a source of wisdom and freedom, of joy and confidence.”
This Christian joy is born from knowing that we are children of God. Saint Josemaría Escrivá used the expression “a joyful affirmation” to underline the deep happiness that comes with discovering yourself as a child of God: “Cheerfulness is a necessary consequence of our divine filiation, of knowing that our Father God loves us with a love of predilection, that he welcomes us, helps us and forgive us” (The Forge, n. 332). Also, joy feeds on the fulfilment of God’s Will: “Accepting the will of God wholeheartedly is a sure way of finding joy and peace” (The Way, n. 758). God’s Will can be in some moments very painful and enigmatic, but those who live by Faith intuit that it is always the best, because “we know that all things work together for good for those who love God, who are called according to his purpose” (Rom 8:28). This is what Saint Thomas More experienced when he wrote to his daughter Margaret from his prison in the Tower of London: “therefore, my own good daughter, do not let your mind be troubled over anything that shall happen to me in this world. Nothing can come but what God wills. And I am very sure that whatever that be, however bad it may seem, it shall indeed be the best.” Saint Josemaría Escrivá said: “For I verily trust in the goodness of God, seem it never so evil to this world, it shall indeed in another even though he may send me suffering. He loves me tenderly, even while wounding me. (…) And I, who also wish to fulfil the most Holy Will of God, following in the footsteps of the Master, can I complain if I too meet sufferings as my travelling companion? It will be a sure sign of my sonship, because God is treating me as he treated his own Divine Son”. (The Way of the Cross, First Station n. 1).
Joy is, thus, compatible with painful circumstances, difficulties and adversities. Since Sainthood consists in an identification with Christ, the Cross is inevitable in Christian life. Even more, Saint Josemaría says that “Joy (…) has its roots in the shape of the Cross”. (The Forge, n. 28).
One early Christian writer says that “for every cheerful man worketh good, and thinketh good, and despiseth sadness; but the sad man is always committing sin”. Since joy is the effect of charity, the hearts of those who seek closeness with God and respond to the call of holiness will consequently overflow with peace and joy: “Each of you must give as you have made up your mind, not reluctantly or under compulsion, for God loves a cheerful giver” (2 Cor 9:7). “Christians are ordinary people, but their hearts overflow with the joy that comes when we set out to fulfil, with the constant help of grace, the Will of the Father.” (Friends of God, n. 93).
“Always happy to make others happy”.
 Pope Francis, Evangelii gaudium, n. 1
 St. Thomas Aquinas, Summa Theologica, I-II, q. 31, a. 3.
 St. Thomas Aquinas, Summa Theologica, II-II, q. 28, a. 4.
 El Pastor de Hermas, Mand X, 3, 2-3 (ed. J.J. Ayán Calvo, Madrid 1995, p. 161). http://www.earlychristianwritings.com/text/shepherd-lightfoot.html