Actually, my sister, the fundamental theological question is not "where does the idea that God only chooses to save certain ones," for only the Universalist, labeled heretics by the entire confessing church believe that God chose to save all. Catholics, Eastern Orthodox, Arminian "Protestants," and even we wicked Calvinists all affirm that God will only save some.
The fundamental question which divides us is upon what basis has God chosen to save.
The answer given by some is that God chose to save based upon some foreseen quality about that individual, whether a baptism performed by men or a decision/ choice made by men.
The answer given by we Calvinists is that God chose to save based upon nothing but his good pleasure to extend mercy to those who would have never chosen Him & don't deserve his grace to begin with. This is based upon numerous passages in the Bible.
Our Presbyterian confession states it thus (other Calvinist confessions use similar language) -- Scripture proofs are footnoted:
V. Those of mankind that are predestinated unto life, God, before the foundation of the world was laid, according to His eternal and immutable purpose, and the secret counsel and good pleasure of His will, has chosen, in Christ, unto everlasting glory,
[9] out of His mere free grace and love, without any foresight of faith, or good works, or perseverance in either of them, or any other thing in the creature, as conditions, or causes moving Him thereunto;
[10] and all to the praise of His glorious grace.
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VI. As God has appointed the elect unto glory, so has He, by the eternal and most free purpose of His will, foreordained all the means thereunto.
[12] Wherefore, they who are elected, being fallen in Adam, are redeemed by Christ,
[13] are effectually called unto faith in Christ by His Spirit working in due season, are justified, adopted, sanctified,
[14] and kept by His power, through faith, unto salvation.
[15] Neither are any other redeemed by Christ, effectually called, justified, adopted, sanctified, and saved, but the elect only.
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VII. The rest of mankind God was pleased, according to the unsearchable counsel of His own will, whereby He extends or withholds mercy, as He pleases, for the glory of His sovereign power over His creatures, to pass by; and to ordain them to dishonor and wrath for their sin, to the praised of His glorious justice.
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VIII. The doctrine of this high mystery of predestination is to be handled with special prudence and care,
[18] that men, attending the will of God revealed in His Word, and yielding obedience thereunto, may, from the certainty of their effectual vocation, be assured of their eternal election.
[19] So shall this doctrine afford matter of praise, reverence, and admiration of God;
[20] and of humility, diligence, and abundant consolation to all that sincerely obey the Gospel.
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