The Concept of 'God': What Does it Mean and, Does it Matter?
Epicurus is correct to see the gods as representing moral ideals. This is the foundation of mythic reality. Within Christianity, for example, the 'imitation of Christ' and discipleship has been prominent. It may have got lost, or been ignored, by some individuals in history, who got caught up religious imperialism.
The question is whether the immortality of God and, any form of mortality, is about symbolism and archetypes, for humans to follow. Even cultural figures, like Jim Morrison, Elvis and Marilyn Monroe may have a 'God-like' symbolic quality, as significant beings who existed and continue to be inspirational icons. Jesus existed as person, and so did Krishna, but the foundation of all gods may not be based on an actual person, or as something living on in the 'heavens'.
As for the metaphysical nature of immortality, beyond the human imagination, it does depend if heaven (and hell) are seen as having an objective foundation. The belief in immortality, beyond role models, depends on how this is seen.
Heaven and hell exist in the human imagination, and may be experienced in this embodied life, but as to whether they are an experiential reality in it's own right is open to question. Some see near death experiences as pointing to such forms of immortality but it is hard to know if they are simply brain states of the person, while still embodied. So, the immortality of God(gods, angels and human spirits) depends on whether there is a dimension beyond embodiment. Ultimately, arguments for and against it are a matter of speculation.