Fr John Flader: Mastery over our thoughts the key

Sins of thought?
I have two questions regarding internal sins. Are sinful thoughts, for example impure thoughts, always sinful? And can these thoughts, for example of lust, be mortal sins?
This is a very important question, on which I believe there is a great deal of confusion. Many people believe they are sinning internally when in fact they are not sinning at all, while others may be sinning mortally without realising it. 
Let us begin by looking at the genesis of internal sins, since it is in this way that we can see the progression from a thought which is not sinful, to a thought which is a venial sin and finally, on occasion, to one which is a mortal sin.
We all have the experience of having thoughts of all sorts pop into our minds unexpectedly and without our having desired them. Some of them are good, such as the sudden remembrance that we need to ring someone, a fond memory of a loved one, or the thought that we are in the presence of God.
Other thoughts are what we would call “bad”, for example a thought of anger against someone who has hurt us, of impure desire, of hatred, etc.
When these involuntary thoughts come while we are praying or engaged in some other good work, we tend to call them distractions, no matter whether they are “good” or “bad”.
The fact of having these undesired thoughts is not, in itself, sinful – or meritorious in the case of the “good” thoughts.
It is simply a manifestation of the fact that we are human, that our imagination is active and it conjures up a constant stream of thoughts.
There is no sin in the case of the bad thoughts, no matter how long they have been in our imagination, unless we consent to them.
Thus, we can be thinking a thought for some time, almost without realising it, and then suddenly become aware that the thought is there and that we should reject it.
If at that point we make the effort to get rid of the thought, to pray for help, to focus our mind on something good, there is no sin at all.
This is the case even if the thought remains in our mind for some time while we try to reject it.
This point is very important since some people confess “having had impure thoughts” when in fact they did not consent to the thoughts and therefore committed no sin.
If, however, when we realise that the bad thought is in our mind we do not make the effort to reject it but rather begin to dwell on it, to take pleasure in it, then we are consenting with our will and there will be sin.
The sin will usually be only venial at first. No matter how vile or serious the thought in itself – such as thoughts of harming someone, of suicide, of hating God, of committing an act of impurity – if we consent to it only for a short time and then reject it, it will not be a mortal sin.
It is simply a matter of human weakness to be slow to reject thoughts that we know are wrong, no matter how bad they may be.
But if we deliberately retain these thoughts for a longer time, if we begin to imagine or plan how we might carry out the act or to fantasise in detail about a particular situation, then we can be sinning mortally.
Jesus’ words in the Sermon on the Mount are very clear: “You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart” (Mt 5:27-28).
And St Thomas Aquinas writes with respect to thoughts of fornication: “When one thinks about fornication and delights in the activity, this occurs because his affections are bent to the act of fornication itself.
“When one consents to this type of delight it is equivalent to consent to affection for fornication … If one deliberately chooses to fix his desire on something that is gravely sinful, it is a mortal sin” (STh I-II, q 74, a. 8).
We should always struggle to control our thoughts and have purity of heart. The book of Sirach says: “Whoever keeps the law controls his thoughts” (Sir 21:11). And Jesus says: “Blessed are the pure in heart, for they shall see God” (Mt 5:8).