Here is a quote from a Catholic source:
**This is the difficulty that St. Paul expresses when he writes: “What shall we say then? Is there injustice with God? God forbid. For He saith to Moses: I will have mercy on whom I will have mercy. And I will shew mercy to whom I will shew mercy.” St. Paul thus answers the difficulty by affirming the principle of predilection, or of the gratuity of grace to which we can have no claim. Further on he states: “O the depth of the riches of the wisdom and of the knowledge of God! How incomprehensible are His judgments, and how unsearchable His ways!” St. Augustine expresses the mystery in these words: “Why He draweth one and not another, seek not to judge if thou dost not wish to err.”
St. Thomas called special attention to these two great difficulties in the mystery of predestination; one difficulty is general in scope, the other of particular interest. He says: “The reason for the predestination of some, and the reprobation of others, must be sought for in the goodness of God. . . . God wills to manifest His goodness in men; in respect to those whom He predestines, by means of His mercy, in sparing them; and in respect of others, whom He reprobates, by means of His justice in punishing them. . . . Yet why He chooses some for glory and reprobates others, has no reason except the divine will. . . . Neither on this account can there be said to be injustice in God, if He prepares unequal lots for not unequal things. . . . In things which are given gratuitously a person can give more or less, just as he pleases (provided he deprives nobody of his due), without any infringement of justice.”
The answers given by St. Paul, St. Augustine and St. Thomas show there is no contradiction. But underlying these two aspects there is an inscrutable mystery because it is essentially supernatural and also because the divine intervention is supremely free. This mystery is supernatural not only modally like a miracle that can be known by natural means, but it is supernatural because by its very nature it belongs to that class of mysteries which concerns the intimate life of God, such as the Trinity, and it thus transcends the natural powers of every intellect whether human or angelic, of every created and creatable intellect. Moreover, in this mystery there is the intervention of God’s supremely free good pleasure, the divinum beneplacitum which St. Paul speaks of. This good pleasure, which is not at all a caprice - for it is the very essence of wisdom and holiness - is for us, as everything is which concerns God’s sovereign liberty, a profound mystery. By this good pleasure God mercifully grants His grace to one of the two thieves crucified with the Savior, whereas in justice He permits the other to resist to the very end, and so lets him remain in sin.**
This is from…
thesumma.info/predestination/predestination6.php
God Bless,
Michael