I am the vine; you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing (John 15:5)
Protestants reject God's predisposing graces and so they can never progress, for all our steps toward God are given to us by God, we practically do nothing. It has nothing to do with intelligence and EVERYTHING to do with God.
Catholic Doctrine on Grace
Outside of the Catholic Church there is no sanctifying grace:
Although, predisposing or prevenient graces are given to anyone of those outside the Catholic Church, so that they can turn to God, change their lives and enter the Catholic Church, for without these graces no one would ever convert. However, outside the Catholic Church no sinner can attain sanctifying grace, the state justice wherein one can receive in their soul the indwelling of the Holy Ghost. They can't receive sanctifying grace because they are in a perpetual state of mortal sin, since there is no way for them to receive remission of sin outside of the Church.
DOGMA:
Pope Boniface VIII, Unam Sanctam, Nov. 18, 1302, ex cathedra: “With Faith urging us we are forced to believe and to hold the one, holy, Catholic Church and that, apostolic, and we firmly believe and simply confess this Church outside of which there is no salvation NOR REMISSION OF SIN…”