The earth erupts, and undead stagger forth. But the cleric brandishes her holy symbol and calls down a flamestrike on the catacomb mouth. The undead are caught in the torrent. In addition to fire damage, they’re subject to holy damage that suffuses her divine spell.
Later, trolls ambush the cleric and her comrades. The cleric directs a beam of searing light into the eyes of the biggest one, potentially blinding it with radiant damage.
The cleric’s warlock friend, sworn to the stars, follows up with the spell glow of Ulban, which spears another troll with a blast of glittering otherworldly light that dazzles it with radiant damage.
And so on. The point of these examples is to illustrate how both radiant damage and holy damage in the same rules might look. Prior to the most recent edition, holy damage was the kind of damage clerics (and other divine characters) dealt when they cast certain good spells. In 4th edition, holy damage was absent, and in its place was radiant damage. However, radiant damage had bigger shoes to fill than just the original concept of holy damage—it also came to be an expression for light charged with uncommon energy, such as particularly intense light, mystical moonlight or starlight, and the alien light of far realms.
The question is, can holy damage and radiant damage exist in the same game?
Some believe the two damage types are mutually exclusive, because radiant damage is holy damage, just with a different name. If this remains true, the damage types do seem like they would be stepping on each other’s toes.
Others think that holy damage should be separate from radiant damage because “good” and “strange/intense light” is too much ground for one damage type to cover. If the rules wanted to contain both damage types, no vestiges of the divine would remain in the radiant type, leaving each to its own sphere.