Is it correct to say that no humans were able to enter into heaven until after Jesus rose from the dead, and if so, why were Moses and Elijah seen in glory with Christ on the Mount of Transfiguration?
Generally speaking, it is correct to say that heaven was closed to man until Christ rose from the dead (cf. CCC 637, 1026), although there is the possibility that exceptions were made in the cases of Enoch (cf. Gen. 5:23-24) and Elijah (cf. 2 Kgs. 2:9-11). As for the Transfiguration (cf. Matt. 17:1-8, Mark 9:2-8, Luke 9:28-36), it is not said that Elijah and Moses were both in heaven, only that they appeared to the apostles who saw them conversing with the glorified Christ. At least in the case of Moses, he may have been in what is now called the limbo of the Fathers (scroll to the section titled “Limbus Patrum”; also see CCC 637).