Never understood this claim by Protestants, to me it's like predestination, just as no one knows for sure if they are one of the predestined, no Protestant can know if they have the "faith", or if they are really have an "internalized active belief", both requisites are undefined. A Protestant can partake in all kinds of mortal sins, as always defined by the Catholic Church, like divorce and re-marriage, use of contraceptives, lying, stealing, hate of his neighbors, even homosexuality (there are actually ordained ministers that preach proudly their homosexuality), etc., and all say that they have "the faith and have internalized active belief".
A Protestant says: "I'm saved by Confession of faith and internalized active belief".
The Catholic Church responds:
Pope Eugene IV, Council of Florence, Cantate Domino, 1441, ex cathedra:
The Holy Roman Church firmly believes, professes and preaches that all those who are outside the Catholic Church, not only pagans but also Jews or heretics and schismatics, cannot share in eternal life and will go into the everlasting fire which was prepared for the devil and his angels, unless they are joined to the Church before the end of their lives; that the unity of this ecclesiastical body is of such importance that only those who abide in it do the Churchs sacraments contribute to salvation and do fasts, almsgiving and other works of piety and practices of the Christian militia productive of eternal rewards; and that nobody can be saved, no matter how much he has given away in alms and even if he has shed blood in the name of Christ, unless he has persevered in the bosom and unity of the Catholic Church.
As far as no one knows for sure if they are one of the predestined, no Protestant can know if they have the "faith", I must disagree completely. Although I do not believe we are born "predestined" for heaven or hell, we certainly can know from Scripture that we have been saved and that we can KNOW we have eternal life. See:
I John 5:13 - I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life.