This is a mana ability. It doesn’t use the stack, and it can’t be responded to.

You can activate this ability even if you have no creature cards with power 5 or greater in your hand, or you have some but choose not to reveal any of them. In those cases, the ability won’t produce any mana.

While you’re casting a creature spell, that card is on the stack, so you can’t reveal it with Sacellum Godspeaker’s ability to produce mana. You can, however, activate this mana ability and reveal that creature card from your hand before you begin to cast the card as a spell.