We should avoid, I think, using the indicative mood for what is really a commandment like the Scout Law ("A Boy Scout is kind to animals" - it means a Boy Scout ought to be kind to animals). For if we hear: "a Christian couple grow in grace and love together" doesn't the question arise "supposing they don't?" It clears the air to substitute the bite of what is clearly a precept for the sweetness of a rosy picture.
Elizabeth Anscombe, Contraception and Chastity (1972).
