In Faith and Hope the world will disagree, / But all mankind's concern is Charity. Pope.
In faith everything depends on "that" you believe; 50 in knowledge everything depends on "what" you know, as well as how much and how well. Goethe.
In fashionable circles general satire, which attacks the fault rather than the person, is unwelcome; while that which attacks the person and spares the fault is always acceptable. Jean Paul.
In ferrum pro libertate ruebant—They rushed upon the sword in defence of their liberty. M.
In flagranti delicto—In the act.
In flammam flammas, in mare fundis aquas—You add fire to fire, and water to the sea.
In for a penny, in for a pound. Pr. 5
In forma pauperis—As a pauper or poor man.
In foro conscientiæ—Before the tribunal of conscience.
In frosty weather a nail is worth a horse. Sp. Pr.