Beatrice, so bright now that Dante must look away, explains the vow with a force that suggests she has been thinking about this with the full weight of her theological training. The vow is the most solemn act a human being can perform: it is the offering of free will itself — the greatest gift God gave — back to God. Nothing can fully replace a broken vow because the thing offered was unique and irreplaceable. One can commute a vow only if a greater good is substituted and only with proper ecclesiastical authority. The Church's keys matter here. Dante's contemporaries who treated vows carelessly — who thought any good work could compensate — were grievously wrong. The vow is not a bargain to be renegotiated; it is a self-donation.
They ascend. The Moon-sphere recedes below them and the Mercury-sphere opens around them, blazing with a new quality of light. More than a thousand souls come rushing toward them, each one brightening as they approach — their inner joy expressing itself as increased luminosity. One speaks, aflame with eagerness to serve Dante's questions. Dante turns to Beatrice to ask permission to speak — and finds her so radiant that he nearly loses himself in contemplation of her beauty. The pattern is established for the canticle: each sphere, Beatrice becomes more beautiful, more radiant, harder to look at, closer to the divine source.