* God Cards are not monsters: The Egyptian God Cards (Slifer the Sky Dragon, Obelisk the Tormentor, and Ra the Mighty) are categorized as Spell Cards in the game. They are not monster cards.
* Monster Reborn only works on monsters: Monster Reborn specifically states that it can only Special Summon a monster from the Graveyard to the field. Since God Cards are Spell Cards, they are not affected by this effect.
Therefore, you cannot use Monster Reborn to bring back the Egyptian God Cards.