Have you no longer kept in mind the words of hope about those whom death has taken from us? "I will take you out of your graves and bring you back to the land of Israel. Then you will know that I am the Lord, when I open your graves and bring you up from your tombs. I will put my spirit within you and you will live. " See Ezekiel 37:11-14 below.
Remember the great Judas Maccabaeus, who wanted to make a sacrifice for the dead, because he rightly believed that they are destined to rise again and that the hour of peace must be hastened for them by timely sacrifices. If Judas Maccabaeus had not been certain of the resurrection, would he have prayed and had others pray for the dead? He, on the contrary, as it is written, thought that a great reward was reserved for those who die piously, as your sons surely did... See the second book of Maccabees 12, 43-46 below.
Book of Ezekiel 37, 11-14
Ezek 37, 11-14 : And he said to me, "Son of man, these bones are all the house of Israel. Behold, they say, 'Our bones are dry, our hope is dead, we are lost! Therefore prophesy and say to them: Thus says the Lord Yahweh: Behold, I will open your graves, and raise you up from your graves, O my people, and bring you back to the land of Israel. And you will know that I am Yahweh, when I open your graves and raise you up from your graves, O my people. I will put my Spirit within you, and you will live; and I will give you rest on your soil, and you will know that I, Yahweh, say and do, - oracle of Yahweh!"
Second Book of Maccabees 12, 43-45
2 M 12, 43-46 : Then, having collected the sum of two thousand drachmas, he sent it to Jerusalem to be used for an expiatory sacrifice. A fine and noble deed, inspired by the thought of the resurrection! For, had he not believed that the soldiers killed in battle would rise again, it would have been useless and futile to pray for the dead. He also considered that a very fine reward is reserved for those who fall asleep in piety, and this is a holy and pious thought. This is why he made this atoning sacrifice for the dead, so that they might be delivered from their sins.