Be what you are and be that well.
Attributed, accurately, to St. Francis de Sales, even though Wikiquote has it on the unsourced page. (I might go create an account and fix that.) A friend pointed me to the source.
It comes from a letter to Madame Marie Bourgeois Brûlartto, wife
of the president of the Burgundian Parliament. Here’s the context:
Do not love anything too much, I beg you, not even virtues, which we sometimes
lose by our excessive zeal. … Let us be what we are and be that well, in order
to bring honor to the Master Craftsman whose handiwork we are. People
laughed at the painter who, intending to paint a horse, came up with a perfect
bull; the work was handsome in itself, but not much credit to the artist who had
had other plans and succeeded in this one only by chance. Let us be what God
wants us to be, provided we are His, and let us not be what we would like to be,
contrary to His intention. Even if we were the most perfect creatures under
heaven, what good would that do us if we were not as God‘s will would have us
Letter CCLXXXIX, dated June 10, 1605, in vol. XIII of the Oeuvres de Saint François de Sales, Évêque et prince de Genève et Docteur de l’Église, edition complète (Annecy: Monastère de la Visitation, 1894), 53-54. Translation by Péronne Marie Thibert, V.H.M. in Francis de Sales, Jane de Chantal: Letters of Spiritual Direction, selected and introduced by Wendy M. Wright and Joseph F. Power, O.S.F.S. (New York: Paulist Press, 1988), 111