Individuals write out their last will and testament as an expression of their will. As a matter of law, their will must be executed exactly as written. Three times in the New Testament epistles, the phrase, “This is the will of God,” occurs (1 Thessalonians 4:3; 5:18; 1 Peter 2:15). These are not suggestions but expressions of God’s will.

But there are other ways of discovering God’s will through His inspired Word—specifically, when commands are given. For example, we discover it is God’s will for us to rejoice by virtue of the Greek grammar in Philippians 4:4. Twice in that verse Paul says to “rejoice,” even saying we are to rejoice “always.” When writing “rejoice” twice in that verse, Paul used the imperative form of the Greek verb. In other words, Paul was commanding the Philippians to rejoice in all things—similar to the same Greek form in 1 Thessalonians 5:18 where Paul said to “give thanks” in everything. These aren’t suggestions; they are the will of God for His people.

If you find it hard to rejoice in a certain situation, you can always rejoice in knowing that God is in control of everything.

We must rejoice in God when we have nothing else to rejoice in.
Matthew Henry