However, the lingering problem is that given God's decision, to save believers and offer of salvation, God obligates Himself to save believers. Given this covenant, God should save believers and it would be morally wrong not to. Consider Hebrews 6:
13 For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself,14 saying, “Surely I will bless you and multiply you.” 15 And thus Abraham, having patiently waited, obtained the promise. 16 For people swear by something greater than themselves, and in all their disputes an oath is final for confirmation. 17 So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it with an oath, 18 so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us. 19 We have this as a sure and steadfast anchor of the soul, a hope that enters into the inner place behind the curtain, 20 where Jesus has gone as a forerunner on our behalf, having become a high priest forever after the order of Melchizedek.
This passage does teach the rock solid nature of God's promises, such that God is somewhat "locked in". Given His promise, He is morally obligated to save believers and if we believe and take God up on the grace He offers, we are putting God in a position where morally He should save us.
So God's promise makes the situation similar to legalism. Per legalism, if we have enough good works, God should give us eternal life. But given God's promise, if we believe, God should give us eternal life.
One line of response it to argue the problem is non-unique. Per Calvinism, believing is our action not God's. Per Calvinism, we are responsible for our actions. So per Calvinism, we are responsible for our faith. However, this only spreads the problem, it doesn't solve it.
Another more promising line of response is that what God promises is mercy and so what He gives is mercy. So salvation is still mercy. This is true, but it seems to create a contradiction rather than solve the problem. God morally has to give something He doesn't morally have to give.
I wonder if open theists or those holding to simple foreknowledge have much more to offer by way of response, but middle knowledge is not done yet.
The real problem is treating God's decision to offer mercy to believers as separable from God's decision to have mercy on Joe and Timmy and Sue, who are believers. While these events are separated by time and logically one helps explain the other - they need not be ontologically separate events in God's mind. God knew who would believe if offered salvation through faith when deciding to offer salvation through faith or not. The objection attempts to prize apart God's unified purpose and pit the elements against each other. But on the other hand, if per simple foreknowledge or open theism, God did not know who would believe if offered salvation through faith, then God was not deciding to save Joe, Timmy and Sue while choosing to offer salvation to believers. Hence, per open theism and simple foreknowledge, conversion does seem to prompt God to save.
William Lane Craig said middle knowledge is one of the more fruitful theological concepts he has come across and he has used it to explain things like perseverance of the saints, inspiration of scripture and Christian particularism. I agree and see it here explaining how God gets all the credit for salvation, even though we believe.