I diagrammed that sentence as I was taught in Catholic school. The subject (noun) is “truth” (singular). The verb should be “is” not “are.” I wish Sister Veronica was still around with her ruler. Whoever wrote that sentence deserves a slap on the back of their hand.
I got years of diagramming in a private American school (Calvert sys) in Istanbul and in 7th grade in Norfolk Va. It has been invaluable to me.