In Romans 9, when Paul is speaking very clearly of God's unconditional election of some, and not others, to eternal salvation, a hypothetical objector to this doctrine raises that very question: “If it is as you say, Paul, and God loved Jacob and hated Esau before they were born, or had done anything good or bad, just so that his own purposes might stand in election, does that not mean he is arbitrary and unjust?” (see Rom. 9:14). Paul's response to this is a resounding, “Of course not! May it never be!” God is not arbitrary or unjust – but he does elect individuals to mercy and hardens others as he sees fit, and for no good will or exertion that he sees in anyone (Rom. 9:15-16). He hardened Pharaoh according to his purpose of displaying his glory in all the earth, and he sovereignly chooses to have mercy on whomever he will, to display the glory of his grace (Rom. 9:17; cf. Rom. 9:22-24). In sum, “Therefore, he has mercy on whom he will and he hardens whom he will” (Rom. 9:18).
We would do well to heed Christ's parable of the workers in the vineyard (Matthew 20:1-16): just because God chooses to have mercy upon some does not make him unjust or arbitrary for giving to others their just deserts. It is his free, undeserved mercy and grace that he holds forth in salvation, and he may do with it as he will. We may not fathom the deep and mysterious ways of God (Rom. 11:33-36); but woe to that one who foolishly says, “I see no reason for why God chooses some and not others, so he must be arbitrary and unjust”. On the contrary, O foolish man, you would do well to say with Job, “Behold, I am of little worth; what shall I answer you? I lay my hand upon my mouth” (Job 40:4).
We would challenge you to wrestle with the following verses. Paul encountered this very same argument against election in Romans 9:18-23; that it would make God unjust and arbitrary:
18 So then He has mercy on whom He desires, and He hardens whom He desires.
19 You will say to me then, "Why does He still find fault? For who resists His will?"
20 On the contrary, who are you, O man, who answers back to God? The thing molded will not say to the molder, "Why did you make me like this," will it?
21Or does not the potter have a right over the clay, to make from the same lump one vessel for honorable use and another for common use?
22 What if God, although willing to demonstrate His wrath and to make His power known, endured with much patience vessels of wrath prepared for destruction?
23 And He did so to make known the riches of His glory upon vessels of mercy, which He prepared beforehand for glory,
To begin with, Paul would not ask this hypothetical question unless He believed the ultimate determination of ones salvation to be in the hands of God alone. Paul is saying that God has the sovereign right to do with us whatever He wants. Will you deny Him this right? This points to an even greater truth: that there is no higher principle in the universe than God Himself. The Hebrew God of the Bible is nothing like the Greek gods which must yield to some greater truth. God is the ultimate Truth and therefore, if He determines something it is, by definition, not arbitrary. In other words, there is no better reason for anything than the fact that God determines it. We should draw no comfort from the theology that promotes a god who must yield to something greater than Himself. Further, since we know the character of God we must not think that, on His side, God had no internal reasons or causes for saving some and not others - - “since the divine purpose always conspires with His wisdom and does nothing without reason or rashly; although these reasons and causes have not been revealed to us. In His counsels and works no cause is apparent, it is yet hidden with Him, so that He has decreed nothing except justly and wisely according to His good pleasure founded on His gracious love towards us.” (Heppe, Reformed Dogmatics) Just because we don’t know His internal reason for choosing some to faith and not others is not reason enough to reject it. In the absence of relevant data, we, therefore, have no reason whatsoever to assume the worse, so there are no legitimate grounds for doubting the goodness of God here. Therefore, to doubt that God can choose us based solely on his good pleasure, is to doubt the goodness of God. The "foreseen faith" people are, in effect, saying that they cannot trust God in making this choice and prefer it to be left up to the fallen individual, as if he would make a better choice than God. This would also make God's love toward us conditional and based on some inherent talent, wisdom or strangth found in the individual rather than in God Himself.