Christian life is zeal and the strength to remain in communion with God by means of an active fulfillment of His holy will, according to our faith in our Lord Jesus Christ, and with the help of the grace of God, to the glory of His most holy name.
The essence of Christian life consists in communion with God, in Christ Jesus our Lord--in a communion with God which in the beginning is usually hidden not only from others, but also from oneself. The testimony of this life that is visible or can be felt within us is the ardor of active zeal to praise God alone in a Christian manner, with total self-sacrifice and hatred of everything which is opposed to this. And so, when this ardor of zeal begins, Christian life has its beginning. The person in whom this ardor is constantly active is one who is living in a Christian way. . . .
Just as salt, penetrating decomposable matter, preserves it from decomposition, so also the spirit of zeal, penetrating our whole being, banishes the sin which corrupts our nature both in soul and body; it banishes it even from the least of the places where it has settled in us, and thus it saves us from moral vice and corruption. . . .
[I]n Christian life the result of the fervor of zeal is a certain quickness and liveliness of spirit, with which people undertake God-pleasing works, trampling upon oneself and willingly offering as a sacrifice to God every kind of labor, without sparing oneself.
Having a firm basis in such an understanding, one may easily conclude that a cold fulfillment of the rules of the Church, just like routine in business, which is established by our calculating mind, or like correct and dignified behavior and honesty in conduct, is not a decisive indicator that the true Christian life is present in us. . . .
Only true zeal both wishes to do good in all fulness and purity, and persecutes sin in its smallest forms.
--The Path to Salvation, pp. 27-28, 29