Be careful and attentive — this is very important — until you see that you are strongly determined not to offend the Lord, that you would lose a thousand lives rather than commit a mortal sin, and that you are most careful not to commit venial sins — that is, inadvertently; for otherwise, who can go without committing many? But there is an advertence that is very deliberate; another that comes so quickly that committing the venial sin and adverting to it happen almost together in such a way that we don’t first realize what we are doing. But from any very deliberate sin, however small it may be, may God deliver us. [I don’t know how we could be so bold as to go against such a great Lord, even though it be in something very small.] What’s more, there is nothing small if it goes against His immense Majesty and we see He is looking at us. It seems to me a sin is very deliberate when, for example, one says: “Lord, although this grieves You, I will do it; I’m already aware that You see it, and I know You do not want it, and I understand this; but I want to follow my whim and appetite more than Your will.” It doesn’t seem to me possible that something like this can be called little, however light the fault; but it’s serious, very serious. [For the love of God, daughters, never become careless in this regard; now — glory be to the Lord — you are not.]

Saint Teresa of Avila
The Way of Perfection, chapter 41