As study of the various mythologies will show,
it is remotely possible for mortals to ascend into the ranks of the divine.
However, there are certain requirements that must be fulfilled before such a thing could happen.

1st,
the character in question must have advanced to an experience level that is significantly above and beyond the average level of adventure-type characters in the general campaign.
(This includes all such non-player types as military leaders, royal magic-users, etc.)
For example, if the average level of characters in a campaign, both player and non-player, is around 5th level, then a candidate for ascension should be something like 9th or 10th level.
If the aveerage level is something like 15th, then a character would have to be in the realm of 25th to 30th level!

Second, his or her ability scores must have been raised through some mighty world-shaking magic to be on par with the lesser demigods.
(Should such an act be lightly considered, remember that a wish spell is the most powerful magic that mankind can control, and such an average increase in abilities would literally take the power of dozens of wishes!
Each use of the spell weakens the caster and ages him 3 years into the bargain, so they are not easy to come by.)

Third, the personage must have a body of sincere worshipers,
people convinced of his or her divinity due to their witnessing of and/or belief in the mighty deeds and miracles which he or she has performed (and continues to perform).
These must be genuine worshipers,
honest in their adoration or propitiation of the person.

Fourth, the person in question must be and have been a faithful and true follower of his or her alignment and patron deity.
It is certain that any deviation will have been noted by the divine powers.

If all of the above conditions have been met,
and the character has fulfilled a sufficient number of divine quests,
then the character's deity may choose to invest the person with a certain amount of divine power,
and bring the character into the ranks of the god's celestial (or infernal) servants.
This process of ascension usually involves a great glowing beam of light and celestial fanfare,
or (in the case of those transmigrating to the lower planes),
a blotting of the sun,
thunder and lightning,
and the disappearance of the character in a great smoky explosion.
Characters thus taken into the realms of the gods will serve their patron as minor functionaries and messengers.
After several centuries of superior service and gradual advancement,
exceptional servants may be awarded the status of demigod,
which includes having an earthly priesthood and the ability to grant spells (of up to 5th level) to the demigod's clerics.

Naturally, ascension to divinity effectively removes the character from the general campaign, as the person will become a non-player member of the DM's pantheon.
The section on DUNGEON MASTERING DIVINE BEINGS applies in general to the summoning of these characters as well,
as their patron deity has more important jobs for his or her servants than to continually send them when called to interfere in worldly matters.

(Should such an act be lightly considered, remember that a wish spell is the most powerful magic that mankind can control, and such an average increase in abilities would literally take the power of dozens of wishes!)
Each use of the spell weakens the caster and ages him 3 years into the bargain, so they are not easy to come by.)

After several centuries of superior service and gradual advancement, exceptional servants may be awarded the status of demigod, which includes having an earthly priesthood and the ability to grant spells (of up to 5th level) to the demigod's clerics.