Modalism means that God is one, but appears in three different "modes" (or masks) instead of being three "Persons" in one Godhead.
When you and the Catholic Catechism say that the Father revealed himself through the Son (or the Spirit) that is Modalism!
If you had said that Jesus revealed the Father to us, that would not be Modalism, since anyone can reveal something about another person. But it means something completely different when you say that one person reveals himself though another person, i.e. by taking on the appearance of that person.
Then said Jesus, Father, forgive them; for they know not what they do. And they parted his raiment, and cast lots. - Luke 23:34
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and [to] my God, and your God. - John 20:21