You being Catholic, I have to be very careful to answer that.
From my Baptist, Protestant, Reformed argument, it is when you kneel in contrition, under conviction by the Holy Spirit, asking Jesus to forgive you of your sins, believing that He is the Son of the Living God, born of a Virgin, died on a cross for your sins, risen on the third day, who now sits on the right hand of the Father.
Now I assume you'll tell me how wrong my answer is.
God Bless
Till all are one.
Of course it is wrong. I'm sorry and I don't mean to be rude, but that is neither the traditional understanding from the very beginning of the Christian faith nor is it in line with the Covenant of God and how He deals with us in a covenant paradigm.
Salvation is being made part of the Covenant Kingdom. In the Old Covenant, we see that the worst curse that could be placed upon a man was that he was "cut off from his people." That is because salvation is corporate, not individual. Salvation is being brought into the Kingdom as a member of the covenant.
And how was that done in the Old Covenant? Circumcision. It made the one being circumcised to be part of the Kingdom of God, which at that time was National Israel.
In continuing along that same principle, baptism replaced circumcision as the ritual of covenant entrance.
Also, salvation is not a "once and done deal" as Evangelicalism teaches. Salvation is a journey, a trek into becoming more and more like Christ. In the Orthodox Church we call this "theosis." The Romans call it "divinization" but it means the same thing - being changed into His likeness more and more and more. Ultimate salvation is nothing less than union with God, which is achieved not by some legal decree of "Not Guilty" in the courts of heaven (Augustine's soteriological idea as refined by the Medieval Catholic Church and Protestantism, i.e. "forensic justification) but by what St. Paul said "putting off the old man of sin and putting on the new man created in Christ." It is an eternity long process which begins here by being made part of the Kingdom by baptism into the covenant.
The people of Christ are covenant people. Jesus said "This is the New Covenant in my Blood." The word covenant is mentioned over 300 times in the Bible, making it a very important principle. Therefore, since our salvation is covenant in nature, it must follow the principles of a covenant.
Go to Amazon and find the book THE DANCE OF ISAIAH by Patrick S. O'Hara. He does a pretty good job of explaining this as a former Protestant turned Catholic.