Celestial Flare — ruling 1

Celestial Flare targets only a player, not any creature. A creature with hexproof or protection from white can be sacrificed this way.

Celestial Flare — ruling 2

If the player sacrifices a blocking creature, any attacking creature it was blocking remains blocked. Unless that attacking creature has trample or is being blocked by another creature, it won’t assign or deal combat damage.

Celestial Flare — ruling 3

Creatures continue to be attacking or blocking creatures through the end of combat step. It is possible to cast Celestial Flare within combat but after combat damage is dealt (specifically, during the combat damage step or the end of combat step). Only attacking and blocking creatures that survived combat damage can be sacrificed at this time.

Celestial Flare : règles, fonctionnement et interactions (MTG)

Celestial Flare is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target player sacrifices an attacking or blocking creature of their choice.

How it works : Celestial Flare

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
Celestial Flare targets only a player, not any creature. A creature with hexproof or protection from white can be sacrificed this way.
No other card is named in this ruling.
#2July 1, 2013· Wizards of the Coast
If the player sacrifices a blocking creature, any attacking creature it was blocking remains blocked. Unless that attacking creature has trample or is being blocked by another creature, it won’t assign or deal combat damage.
No other card is named in this ruling.
#3July 1, 2013· Wizards of the Coast
Creatures continue to be attacking or blocking creatures through the end of combat step. It is possible to cast Celestial Flare within combat but after combat damage is dealt (specifically, during the combat damage step or the end of combat step). Only attacking and blocking creatures that survived combat damage can be sacrificed at this time.
No other card is named in this ruling.