To unravel the mystery of predestination is impossible for us, but try we do in order to understand good and evil in the world.
The basic truth is this: God gives grace, but we choose whether to use that grace with our free wills or not.
But, grace is efficacious "only as regard the elect". Garrigou-Lagrange outlines more points for us, using the sources on pages 19-20.
"Whatsoever the Lord pleased he hath done in heaven and on earth. For nothing is done in heaven or on earth, except what He Himself is pleased to do, or justly permits to be done....(2) God wills all men to be saved and no on to perish...nor after the fall of the first man is it His will forcibly to deprive man of free will. (3) That those, however, who are walking in the path of righteousness in their innocence, He heals and aid their free will by grace. (4) The who go far from God, who is desirous of gathering the children of Jerusalem that will it not, will perish. (5) Hence it is because of God's grace that the world is saved; and it is because man has free will that the world will be judged. (6) Adam, through willing what was evil, lost the power to do what is good....Wherefore the whole human race became a mass of perdition. If no one has been rescued from it, God's justice would not have been to blame. That many are saved, however, is due to God's ineffable grace."
Council of Thuzey
Of course, the Council of Trent had to revisit this topic because of Protestantism and Jansenism-the subjects of Part Six.