The Secret of Sanctity (St. Francis de Sales)
St. Francis de Sales (1567 - 1622) was a Bishop of Geneva, noted for his deep faith and gentle approach to the religious divisions that came from the Protestant Reformation. He is known also for his writings on spiritual direction and spiritual formation, in particular the Introduction to the Devout Life, the Treatise on the Love of God and this work which speaks of how to become more holy.
From the Preface:
Our great happiness as Christians is to possess, in this world, through grace and love, Him Who deigns to be our beatitude for all eternity; and our greatest misfortune, after sin, is not to know or to recognize this secret of eternal charity. God would have us holy even as He is holy; He would have us live His very life. It is for this end that He has given us His divine Son, and with Him the infinite riches of His heart; that is, His merits, His sacraments, His Church. Sanctity consists in believing and receiving these divine communications, of which Jesus Christ is the source, the instrument, and the end; consequently, it also consists in uniting ourselves with Him by loving Him, and in modelling ourselves upon Him by imitating Him; it can and ought to pervade every life, the busiest as well as the simplest.