Adam Clarke on 1 Thessalonians 4:14

For if we believe that Jesus died and rose again - Ει γαρ· Seeing that we believe; knowing that the resurrection of Christ is as fully authenticated as his death.

Even so them - It necessarily follows that them who sleep - die, in him - in the faith of the Gospel, will God bring with him - he will raise them up as Jesus was raised from the dead, in the same manner, i.e. by his own eternal power and energy; and he will bring them with him - with Christ, for he is the head of the Church, which is his body.