What if someone makes such a plan and then repents and does not act on it? Are they still in sin?
ICXC NIKA
If the thought was mortally sinful, then it must be confessed in the sacrament of confession. The three factors must be considered to determine mortal or venial sin.
(Baltimore Catechism No. 3)
Q. 282. How many things are necessary to make a sin mortal?
A. To make a sin mortal, three things are necessary: 1. a grievous matter, 2 sufficient reflection, and 3 full consent of the will.
Q. 283. What do we mean by “grievous matter” with regard to sin?
A. By “grievous matter” with regard to sin we man that the thought, word or deed by which mortal sin is committed must be either very bad in itself or severely prohibited, and therefore sufficient to make a mortal sin if we deliberately yield to it.
Q. 284. What does “sufficient reflection and full consent of the will” mean?
A. “Sufficient reflection” means that we must know the thought, word or deed to be sinful at the time we are guilty of it; and “full consent of the will” means that we must fully and willfully yield to it.
Does this sound familiar:
I confess to almighty God
and to you, my brothers and sisters,
that I have greatly sinned,
in my thoughts and in my words,
in what I have done and in what I have failed to do,
through my fault, through my fault,
through my most grievous fault;
therefore I ask blessed Mary ever-Virgin,
all the Angels and Saints,
and you, my brothers and sisters,
to pray for me to the Lord our God.