The Letter to the Friends of the Cross is largely a meditation on the words of Christ:
If anyone wants to be a follower of mine, let him renounce himself, and take up his cross and follow me (Mt 16:24; Lk 9:23).
It is addressed to the members of an association which he seems to have established in various places during the course of his missions, and presents the self-renunciation called for by Christ as a necessary, beneficial and glorious means of becoming a true disciple of Christ. St Louis Marie here gives practical "rules" for making our own sufferings and acts of penance and mortification a real "following of Christ". It could well be read in conjunction with what he says in The Love of Eternal Wisdom on the third means for acquiring Divine Wisdom: universal mortification.