Celestial Crusader

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2021-03-19
  Official Ruling
  If the resolution of a triggered ability involves casting a spell, that spell can't be cast if a spell with split second is on the stack.
• WWizards of the Coast2021-03-19
  Official Ruling
  After a spell with split second resolves (or otherwise leaves the stack), players may again cast spells and activate abilities before the next object on the stack resolves.
• WWizards of the Coast2021-03-19
  Official Ruling
  Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a white creature may become lethal if Celestial Crusader leaves the battlefield during that turn.
• WWizards of the Coast2021-03-19
  Official Ruling
  Split second doesn't stop triggered abilities from triggering, such as that of Chalice of the Void. If one does, its controller puts it on the stack and chooses targets for it, if any. Those abilities will resolve as normal.
• WWizards of the Coast2021-03-19
  Official Ruling
  Players still get priority while a card with split second is on the stack; their options are just limited to mana abilities and certain special actions.
• WWizards of the Coast2021-03-19
  Official Ruling
  Players may turn face-down creatures face up while a spell with split second is on the stack.
• WWizards of the Coast2021-03-19
  Official Ruling
  Casting a spell with split second won't affect spells and abilities that are already on the stack.

Rulings via Scryfall