Praying for the dead is quite biblical. In the Book of Maccabees found in the Catholic and Orthodox Old Testament, Judas Maccabees took a collection for a sin offering for his men who died in battle: ...

Catechism of the Catholic ChurchSecond EditionPart TwoThe Celebration of the Christian Mystery Section TwoThe Seven Sacraments of the Church CHAPTER ONE THE SACRAMENTS OF CHRISTIAN INITIATION ...