"But I do not want you to be ignorant, brethren, concerning those who have fallen asleep, lest you sorrow as others who have no hope. For if we believe that Jesus died and rose again, even so God will bring with Him those who sleep in Jesus."
As we approach Memorial Day weekend, this is a good reminder for believers. If we've lost loved ones who also knew the Lord, we have the promise of God's Word that He will one day resurrect their bodies, just as He resurrected the body of Jesus. (To be reunited with their souls, which went to Paradise the instant of their physical death.)
Thus, we are not to "sorrow as others who have no hope." That's not to say Christians shouldn't grieve the loss of a loved one. Grieving is natural and part of the healing process. Even Jesus cried when His friend Lazarus died. However, it is to say we shouldn't grieve in the same way as a lost person would grieve. Why? Because as a follower of Jesus, we know we'll see our Christian loved one again. For the Christian, death is not the end. It is only a temporary separation.