On the Memorial Saturday before Great Lent begins, Fr. Matthew Howell gives a couple of reasons why we pray for the souls of those who have died. He cites examples from 2 Maccabees 12:38-45 and from Empress Theodora in 843 AD.
1 Thessalonians 4:13-17
But we would not have you ignorant, brethren, concerning those who are asleep, that you may not grieve as others do who have no hope. For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep. For this we declare to you by the word of the Lord, that we who are alive, who are left until the coming of the Lord, shall not precede those who have fallen asleep. For the Lord himself will descend from heaven with a cry of command, with the archangel’s call, and with the sound of the trumpet of God. And the dead in Christ will rise first; then we who are alive, who are left, shall be caught up together with them in the clouds to meet the Lord in the air; and so we shall always be with the Lord.
Luke 21:8-9; 25-27; 33-36
The Lord said, “Take heed that you are not led astray; for many will come in my name, saying, ‘I am he!’ and, ‘The time is at hand!’ Do not go after them. And when you hear of wars and tumults, do not be terrified; for this must first take place, but the end will not be at once. And there will be signs in sun and moon and stars, and upon the earth distress of nations in perplexity at the roaring of the sea and the waves, men fainting with fear and with foreboding of what is coming on the world; for the powers of the heavens will be shaken. And then they will see the Son of man coming in a cloud with power and great glory. Heaven and earth will pass away, but my words will not pass away. But take heed to yourselves lest your hearts be weighed down with dissipation and drunkenness and cares of life, and that day come upon you suddenly like a snare; for it will come upon all who dwell upon the face of the whole earth. But watch at all times, praying that you may have strength to escape all these things that will take place, and to stand before the Son of man.”