The work of Christ actually accomplishes redemption for His elect alone

The Bible through out presents to us the gospel, no doubt there is law in the Bible as well by which on condition of completing the commands a man may attain the promises, but the testimony of the Bible is that all are sinners and fall short of the glory of God; the gospel, revealed throughout the Bible is presented to us as the accomplished redemption of what Christ in fact got done on the cross for His elect people alone. This is the good news that by Christ's merit and righteousness alone, imputed to the elect sinner received by faith alone are justified and accepted by God. There is no works which God's people do or can do that either makes or breaks this acceptance.
The death of Christ and the His merits imputed to the elect are so intertwined that those whom he represented and died for are the very ones that in time do/will in fact believe. To teach that Christ's death is meant for everyone of Adam's children head for head and that this death is only received by so many of Adam's posterity is to teach basically that Christ's death is not itself what actually saves people but that those who are saved and those who are not saved owe their salvation to something else whether that be by the Spirit's power or ours.

Determinism

"There are some Christians, even some who have been raised in Presbyterian homes and churches, who dislike the idea of determinism. These people, to varying degrees, have been infected with the Arminian notion of a free will, a will independent of God. But the Confession is not afraid of determinism. Note carefully that the effectual or effective call occurs by God's 'renewing their wills, and by his almighty power determining them to that which is good, and effectually drawing them to Jesus powerful than any alleged physical determinism. the sinner comes 'freely,' i.e. voluntarily, not in spite of but because of the fact that God controlled his will and made him willing.
The reason it is so necessary to insist on divine determinism and to rebut any doctrine of a will free and independent of god's causative power is that man since the fall is dead in sin, cannot will to accept Christ, and is wholly dependent on grace. Thus section ii not only ties in with section I, but also depends on chapter VI." - Gordon H. Clark, What Do Presbyterians Believe?