While we say that we "fall into sin," there must always be a free choice for us to be culpable.
In the English language, Catholics will often talk about the idea of falling into sin. At face value this could appear to mean that somehow we end-up in sin, without us even realizing what took place.
It’s as if we fell into a hole of sin, not looking where we were going.
To a certain degree, there can be some truth to that statement. We can be led into sin unexpectedly through our actions.
For example, maybe we saw a scandalous ad when surfing the internet, and accidentally clicked on it. Initially we would not be at fault if we clicked on it by accident. However, if we choose to stay there and interact with the content of the website, then we would be committing a sin.
The Catechism of the Catholic Church explains how at the root of every serious sin is a free choice.
Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice.
To choose deliberately — that is, both knowing it and willing it — something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin. This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death.CCC 1861, 1874
On the other hand, unintentional ignorance can remove our culpability, especially if it was a true and accident, and not something that we desired to commit in any way.
Unintentional ignorancecan diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.CCC 1860
God inscribed the moral order into our heart, so even if we don’t know all the laws of the Church or even the Ten Commandments, we can still have an innate sense of what is good or evil.
It is up to our conscience at the time of the sin to know if what we committed was a free choice, or if it was unintentional ignorance.
Falling into sin may be possible, but if we truly do sin against God, our action has to be free and not something that we do by accident. We need to take responsibility for our actions and if we are in a habit of sin, we need to break free from that addiction and experience the beautiful mercy of God.