The key figure here is Jesus. For him, not the word “koimenon” (to sleep) is used, but the Greek word “apethanen” (passed away). As Leon Morris says: He died that death which is the wages of sin; and because He endured the full horror implied in that death, He has transformed death for His followers into sleep
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And his atoning death is accepted by God. For, behind Jesus’ whole life and death was God’s hand. On the third day after his death on the cross, God raised him from the grave. That is why this mighty and gracious God will surely arrange that also the deceased believers will join Jesus on the great day of his return (1 Thessalonians 1:10, 1 Thessalonians 2:19).
14 For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep.