We can be joyful at all times not because we are spared affliction but because we are completely satisfied in Christ.
Jesus promises that we will suffer. In Matthew 10:17-18, Jesus warns his followers, “Be on your guard against men; they will hand you over to the local councils and flog you in their synagogues. On my account you will be brought before governors and kings as witnesses to them and to the Gentiles.” Although that quote may seem distant or irrelevant to Christians today, the point remains that, in the words of Christ, “if the head of the house [Jesus himself] has been called Beelzebub, how much more the members of his household!”
Not only does Jesus promise that believers must suffer, the Apostle Paul assures us that the path to being an heir of God demands sharing in the sufferings of Christ (Romans 8:17). Similarly, Paul writes this to those in the church at Philippi: it has been granted to you on behalf of Christ not only to believe on him, but also to suffer for him (Philippians 1:29).
Clearly, we, as Christians, will not be spared affliction. (In addition to the Scriptures above, see Acts 9:15-16, Philippians 3:10-11, and 1 Timothy 1:8-9.)
Despite being assured affliction and suffering, Christ-followers can be joyful nonetheless.
We can be joyful not by creating a bubbly-faced facade of happiness to hide reality but rather by confidently knowing that, in the face of immense sin and suffering, we are secure in Christ. Why fear and be saddened by those who can only kill the body when you’re protected by Him who can keep you from hell (Luke 12:4-7)? Or, to put it positively, rejoice that you have received the Holy Spirit, who guarantees your eternal salvation—and thus, your eternal joy (2 Corinthians 1:21-22).
The faith that justifies is the very same faith that satisfies. Saving faith satisfies us by weaning us from the short-lived satisfaction of sin to ultimate satisfaction in Christ.
Thus, despite the fact that suffering will come even—or, perhaps, especially—to us who profess faith in Christ, we can have great joy not because we are spared affliction but because we are completely satisfied in Christ.