St. Peter Faber was one of the greatest of the early Jesuits, who was praised by St. Ignatius himself as the greatest giver of his Spiritual Exercises. His ministry was done in many areas where Protestantism was rampant and, therefore, it was a practical necessity that he often work towards the conversion of heretics to the Catholic Faith. On May 7th, 1546, he wrote a letter to Jacobo Lainio with some practical advice to this end. Here, I present an English version of the Letter.
NOTE: If you are interested in the teaching of St. Ignatius on spirituality more broadly, consider picking up a copy of the Brief Guide to Mental Prayer According to the Mind of St. Ignatius.
Letter 138
He Advises Loving Them in Truth
The first is, that whoever wants to benefit the heretics of this time, must aim to have much charity towards them and love them in truth (in veritate), discarding from their spirit all the considerations that tend to cool their esteem for them.
Make an Effort So That We Are Loved by Them
It is necessary to win them over so that they love us and hold us in good esteem within their spirits; this is done by communicating with them familiarly on things that are common to both of us (nobis et ipsis), avoiding all disputes (disceptationes), where the other side (altera pars) seems to suppress the other; for it is first necessary to communicate on those things that unite, rather than on those that seem to show a diversity of opinions.
With Heretics, It Is More Beneficial to Direct the Will than to Teach the Intellect About Faith; Which Is Done Differently with Pagans
Because as much as this Lutheran sect are children of withdrawing unto perdition (Heb. 10:39), and first the good sense (i.e., will) was lost, not the good belief, in them, it is necessary to proceed from those things that are and are valuable for feeling rightly according to affection (affectum), to those that are for believing rightly; contrary to what was done at the entry of the early converts to the Faith; for there it was first necessary to teach and correct the understandings through the way of faith, which is of hearing (Rom. 10:17), and from there to come to feel rightly about doctrine and good works (de operibus), which are acceptable once they [believe] according to the faith once accepted. (cf., Jude 3, sunt secundum fidem semel acceptam).
They Must First Be Drawn Away from Vices Rather than from Errors, as If Through Tunnels
Taking one by the hand, who is not only of bad and perverse doctrine but also of bad life, it is necessary to find ways with such a one, to be able to dissuade and remove the vices of his bad life, before this, not speaking to him about what concerns his errors about the Faith. It has happened to me, for example, for one to come to ask me to satisfy him about some errors he had, especially about priestly marriage (coniugio sacerdotum), and I, communicating with him in such a way that he communicated his life to me, which was in a state of mortal sin, for being with a concubine for many years, made it so that he left such a life, without entering into dispute about what touched upon the Faith; and then, separated from sin and seeing himself free to be able to live without the woman with the grace of the Lord, his errors dissolved, without speaking more on them, because they depended on that bad life.
One Must Proceed from Good Works to the Faith Itself Regarding Precepts
In the errors they have concerning good works (circa operabilia), which is quasi universal in this sect, it is necessary to proceed from the works themselves to the Faith itself, always speaking to them of things that can induce them to love the works; for example, when someone among them says that the Church cannot oblige under pain of moral sin to say the Divine Office, hear Mass, etc. (ecclesiam non posse obligare sub pena peccati mortalis ad dicendum officium, ad audiendum sacrum), it is necessary to use exhortation to move their soul to prayer and to the work, which is hearing mass; because this person first lost the devotion of hearing mass or of praying vocally with order and limitation, afterwards, by logical progression (vero), [they] lose the Faith.
Their Spirit Must Be Nourished with a Great Fervor, So That They May Be Well Offered
It is very important to note where the leaders among them base their errors, which are contrary to the precepts and ordinations of the Church and Holy Fathers (qui sunt contra precepta et ordinationes ecclesiae et sanctorum patrum), finding themselves so weak to obey and suffer, that it seems to them impossible to keep the precepts and rules; therefore, exhortation of spirit is necessary to strengthen and encourage them, so they may gain hope of being able to do it and to suffer what is commanded and more, with the grace of the Lord; here I say that whoever via doctrine and fire of spirit could persuade Luther to leave what he has and to submit with obedience to do what they command him, taking up the habit that he left, due to this alone (hoc ipso facto), he would cease to be a heretic, without another dispute. True is that it would be necessary to have forces of spirit to strengthen him; it would be necessary to have much fire to make him enter into such humility, into such patience, and for all the other virtues (et sic de aliis virtutibus), which are necessary for a man in such a state, as I said, passing through such a fall and such a subversion; and since it is difficult or impossible without the finger of God for such things to be done in men who have been subverted, therefore it is not easy to hope for the reduction of these heretics.
A Wise Man, by Speaking to Heretics About the Right Way of Living, Will Obtain More from Them, than a Scholar Overwhelming and Confusing Them with the Weight of Doctrine
Whoever could not speak with them except in a way of right living (recte vivendi), of virtues (virtutibus), of prayer (oratione), of death (morte), of hell (inferno), and similar things that contribute to the amendment of even the lives of pagans, would benefit them more than anyone else filled with authorities to confound them.
They Must Finally Be Encouraged Towards Fear and Love of God; And God Must Be Prayed To, So That He May Grant Them Help
In summary, these people need admonitions, exhortations, etc., regarding manners, regarding the fear and love of God and of good works, to counter their weaknesses, their lack of devotion, distractions, burdens they bear, and other evils, which are not primarily, nor firstly, in the understanding, but in the feet and hands of the soul and body.