Semi-pelagianism, like Arminianism, allows that God's grace must operate on a man...but is operating on all men (common grace). Then, with this common (prevenient) grace at work, the man is given the choice to follow Christ or reject Christ. This "little island of righteousness", just a tiny atoll, is sufficient to grant the man enough wherewithal to accept or reject the grace needed to save. Although it was also condemned (centuries ago), it has become the theology de jure with Rome today. And now this errant doctrine is not just a problem with Rome...it has permeated much of the so-called Protestant movement.
As Pascal said, man is a reed, but a thinking reed.
Wow, how many errors can you pack into one paragraph? You don't understand either semi-Pelagianism or Catholicism.
"Prevenient" grace is not the same thing as "common" grace. "Prevenient" comes from the Latin meaning "coming before". It is the grace which precedes justification.
Semi-Pelagians taught that the initiative in the ordo salutis belongs to man; that a "good thought" or "good movement" in a man could begin the process of justification. The doctrine of prevenient grace directly opposes this by insisting the the initiative always belongs with God.
The Catholic church condemned semi-Pelagianism at the Council of II Orange, whose canons were given dogmatic force by a later Papal degree. Those canons retain their dogmatic force today.
Semipelagianism is the belief that man, by following the example of Christ, can do good works that will somehow earn him, or entitle him, to God's grace of salvation.
Calvinism is the belief that man cannot do good even with the help of God's grace - that grace consists of God forcibly making the elect to perform actions acceptable to Him by an "irresistible" compulsion.
Orthodox Christianity holds that man is deeply flawed by his very nature due to original sin and that he can accomplish no good thing unless God gives him the grace to do so. It likewise holds that grace is a free gift of love and that God asks man to cooperate with His grace - not because He needs man to do so, but because it pleases Him. In other words, we should be slaves (in Calvinist fashion) but He makes us sons by adoption.