We must maintain the following about the source of the sacraments: that they are sensible signs divinely instituted as remedies in which, "under the cover of material realities, divine power operates in a hidden manner." Thus, "they represent by similitude, signify by their institution, and confer a certain spiritual grace by sanctification" through which the soul is cured from the weakness of its vices. They are principally ordained to this as their final end; but as subordinate ends, they also are a means of humiliation, instruction, and exercise.
[St. Bonaventure, Breviloquium, Monti, ed. and tr., Franciscan Institute Publications (Saint Bonaventure, NY: 2005) pp. 211-212. The quotations are from St. Isidore (Etymologies 6.19.40) and Hugh of St. Victor (On the Sacraments 1.9.2).]
I've decided this Lent to make sacraments the theme.