A body buried in a wooden casket would normally be entirely decomposed after a few hundred years, depending upon the conditions of the soil. Similarly, a seaman buried at sea would leave no traces. (Not a trace seems to remain of all of those who went down with the Titanic, for instance.)
The apostle Paul made it clear that our new body, though possessing some identity with our mortal body, will be a new “spiritual body” ( 1 Corinthians 15:35-44 ). God will not need to gather up the scattered molecules of our earthly bodies. (Remember that the bodies of many Christians have already decomposed, been completely destroyed by fire, or have been devoured by animals.) Therefore, 1 Thessalonians 4:13-18 doesn’t refer to a bizarre scene in which the ashes in funerary urns or decayed bodies in earthly graves are suddenly reconstituted. Rather, the resurrection is the wonderful occasion in which believers who have died will again be granted full bodily form, this time in a glorified heavenly body that can never again die or experience decay.