Why can Christians today be present with the Lord at death, while Old Testament believers could not?
Christians can be absent from the body and present with the Lord because Christ's resurrection took paradise to heaven. Old Testament believers, except Enoch and Elijah, went to paradise in Hades, unable to enter heaven, because no atoning bloodshed had occurred; animal blood could not remove sin, serving only as a stopgap.
Les explains that throughout the Old Testament economy, even believers—except Enoch and Elijah—did not go to heaven when they died. They went down into paradise, which he identifies with the paradise side of Hades or hell, illustrated by Abraham and Lazarus and the rich man in torment, with a great gulf fixed. Jesus descended into that paradise side, and on His resurrection morning He took paradise to heaven. Now Paul tells us that to be absent from the body is present with the Lord. In the Old Testament this was not possible because there had been no atoning bloodshed; the blood of animals and goats could not take away sin, so it was only a stopgap.