Speaking at the launch of his new book "Cain", Jose Saramago, who won the 1998 Nobel Prize for Literature, said society would probably be better off without the Bible.
Roman Catholic Church leaders accused the 86-year-old of a publicity stunt.
The book is an ironic retelling of the Biblical story of Cain, Adam and Eve's son who killed his younger brother Abel.
At the launch event in the northern Portuguese town of Penafiel on Sunday, Saramago said he did not think the book would offend Catholics "because they do not read the Bible".
"The Bible is a manual of bad morals (which) has a powerful influence on our culture and even our way of life. Without the Bible, we would be different, and probably better people," he was quoted as saying by the news agency Lusa.
Saramago attacked "a cruel, jealous and unbearable God (who) exists only in our heads" and said he did not think his book would cause problems for the Catholic Church "because Catholics do not read the Bible.
"It might offend Jews, but that doesn't really matter to me," he added.
Father Manuel Marujao, the spokesman for the Portuguese conference of bishops, said he thought the remarks were a publicity stunt.
"A writer of Jose Saramago's standing can criticise, (but) insults do no-one any good, particularly a Nobel Prize winner," the priest said.
Rabbi Elieze Martino, spokesman for the Jewish community in Lisbon, said the Jewish world would not be shocked by the writings of Saramago or anyone else.
"Saramago does not know the Bible," the rabbi said, "he has only superficial understanding of it."
The author caused a scandal in Portugal in 1992 with "The Gospel According to Jesus Christ."
The book depicted Jesus losing his virginity to Mary Magdalene and being used by God to control the world.
Saramago quit Portugal at the time and moved to Lanzarote, in the Spanish Canary Islands.
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