1 Thessalonians 4:13 But we do not want you to be ignorant, brothers, concerning those who are sleeping, that you would not grieve even as also the rest who have no hope.
14 For if we believe that Jesus died and rose, so also those who have fallen asleep through Jesus, God will bring with Him.
John 11:11 He said these things, and after this He said to them, Our friend Lazarus has fallen asleep; but I am going that I may wake him out of sleep.
12 The disciples then said to Him, Lord, if he has fallen asleep, he will recover.
13 But Jesus had spoken about his death, but they thought that He was speaking about taking rest in sleep.
14 So Jesus then told them plainly, Lazarus has died.
Those who are “sleeping” as pointed out in the verses above, refer to the believers of the Lord who have died. But the Lord did not say that were dead, he said that they were sleeping. Meaning that they will be awakened by the Lord at his coming back and will be raised from the dead. The Lord said the same thing concerning Lazarus. He referred to Him as one who has fallen asleep. All of the dead believers will rise first when the Lord Jesus comes back (1Thes. 4:15).