The source of this is the union of the two natures in Jesus Christ; and also the two worlds (the creation of a new heaven and a new earth; a new life and a new death; all things double, and the same names remaining); and finally the two natures that are in the righteous, (for they are the two worlds, and a member and image of Jesus Christ.  And thus all the names suit them:  righteous, yet sinners; dead, yet living; living, yet dead; elect, yet outcast, etc.).

There are then a great number of truths, both of faith and of morality, which seem contradictory, and which all hold good together in a wonderful system.  The source of all heresies is the exclusion of some of these truths; and the source of all the objections which the heretics make against us is the ignorance of some of our truths.  And it generally happens that, unable to conceive the connection of two opposite truths, and believing that the admission of one involves the exclusion of the other, they adhere to the one, exclude the other, and think of us as opposed to them.  Now exclusion is the cause of their heresy; and ignorance that we hold the other truth causes their objections.

1st example:  Jesus Christ is God and man.  The Arians, unable to reconcile these things, which they believe incompatible, say that He is man; in this they are Catholics.  But they deny that He is God; in this they are heretics.  They allege that we deny His humanity; in this they are ignorant.

2nd example:  On the subject of the Holy Sacrament.  We believe that, the substance of the bread being changed, and being consubstantial with that of the body of our Lord, Jesus Christ is therein really present.  That is one truth.  Another is that this Sacrament is also a type of the cross and of glory, and a commemoration of the two.  That is the Catholic faith, which comprehends these two truths which seem opposed.

The heresy of to-day, not conceiving that this Sacrament contains at the same time both the presence of Jesus Christ and a type of Him, and that it is a sacrifice and a commemoration of a sacrifice, believes that neither of these truths can be admitted without excluding the other for this reason.

They fasten to this point alone, that this Sacrament is typical; and in this they are not heretics.  They think that we exclude this truth; hence it comes that they raise so many objections to us out of the passages of the Fathers which assert it.  Finally, they deny the presence; and in this they are heretics.

3rd example:  Indulgences.

The shortest way, therefore, to prevent heresies is to instruct in all truths; and the surest way to refute them is to declare them all.  For what will the heretics say?

In order to know whether an opinion is a Father’s ...

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All err the more dangerously, as they each follow a truth.  Their fault is not in following a falsehood, but in not following another truth.