Good to know that Duolingo is anti-Donatist.
Saint Augustine approves! ☺
I was about to say the same...
Deceitful is mendax mendacis. Perfidius, perfidi is dishonest false etc
Mendax is closer to "liar," although I agree that perfidus would be better translated as "faithless" or perhaps "untrue" in the original sense of someone who will not uphold an oath.
Sacrificare postest should work
Why doesn't sacrificant work for the plural of sacrifice?
It can, but not here where you have to use the infinitive. If it were several priests then you would say sacerdōtēs perfidī sacrificāre possunt, conjugating possum instead of sacrificō.