If I had a {heartless summoning} in play then played a {knight of glory}, would it be subject to the -1/-1 from the summoning?
Yes, knight of glory would still be affected by {Heartless Summoning}, including the -1/-1.
Protection stops DEBT
Damage
Enchanting
Blocking
Targeting
Since this -1/-1 is just a global effect that is/does none of the above, protection will not stop it.
RESOLVED
Thanks for the help, Gorzo!