"At the heart of the division between Protestants and Catholics is the disagreement over the doctrine of justification. According to Catholic theology, justification is believed to be a lifelong process in which the sinner becomes intrinsically righteous, to the extent that he or she cooperates with grace. On the other side, Protestants assert that this ends up confusing justification with sanctification. Whereas sanctification is a process, justification is a once-for-all declaration of right standing with God that is ultimately rooted in Christ’s perfect righteousness."
Understanding the Protestant / Catholic Divide
The Church no longer offered certainty of salvation; she had become questionable in her whole objective form—the true Church [therefore] had to be sought outside the institution. It is against this background…that we are to understand that Luther, in the conflict between his search for salvation and the tradition of the Church, ultimately came to experience the Church, not as the guarantor, but as the adversary of salvation.
JOSEPH CARDINAL RATZINGER (LATER POPE BENEDICT, XVI), PRINCIPLES OF CATHOLIC THEOLOGY, P. 196.
Understanding the Protestant / Catholic Divide