You are merging Justification with Sanctification. Belief alone in the finished work of Christ is salvation. How much faith is needed? Any more than none at all. Remember, the Holy Spirit convinces the unbeliever that he is lost, and makes it clear that faith alone is salvation.
Faith plus works is Sanctification, becoming more like Christ.
When we discuss the Gospel with those in the Roman Catholic Church I think we must remember their policies are not at all in accord with those of the Protestant.
Roman Catholics are subject to infant baptism. Known as, Paedobaptism. They're basically to adhere to church digmas from infancy.
Jesus never taught infant baptism.
Also,it may help to know that Roman Catholics cannot agree with the faith alone teaching of Jesus.
Because the church commissioned the Ecumenical Council of Trent to respond to the Protestant Reformation.
The church codified Dogmas, and corrected in its official decrees those teachings pronounced by the Reformation proponents so to issue Catholic decrees that are thought superior.
And,they defined Paedobaptism. Which they decreed was the only means of salvation for babies.
Catholic are made adherents of the church shortly after birth.
The Council of Trent in 1547 during its sixth session Decreed Justification by faith alone to be anathema, condemned.
Another of the anathemas is belief in Christ alone.
But,in 1983 Canon Law of the Church, the church changed Anathema to the term, Heresy.
While still condemning Justification by Faith alone.
Not the only Anathema on their list of course.
There are other key factors,teachings of Jesus,pronouncements,that the church decreed Anathema,aka Heresy.
All of which still prosecutes violation,such as if a Roman Catholic did agree with Justification by Faith alone, to remain in force.
And that punishment is known as excommunication.
Which is why
@faithhopecharity will
never be moved by our defense of the faith from the Protestant perspective.
A great resource to learn more about the Council of Trent is the book,
Trent: What Happened at the Council
Written by John W. O'Malley.