Then why are believers who have died called "the dead in Christ"?
1 Thessalonians 4:16-18 For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God. And the dead in Christ will rise first. Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord. Therefore encourage one another with these words.
Because the Bible is not a catechism. Words are not used throughout the Bible with a single, univocal, fixed sense.
The “dead in Christ” are DEAD—in the normal, everyday sense of the word. But they partake in eternal life, so they are alive.