Bruce explains the effect of the rewards of God:
These great and precious promises, if believed, would make sacrifices easy. Who would not part with a fishing-boat for a throne? and what merchant would stick at an investment which would bring a return, not of five per cent., or even of a hundred per cent., but of a hundred to one?
The promises made by Jesus have one other excellent effect when duly considered. They tend to humble. Their very magnitude has a sobering effect on the mind. Not even the vainest can pretend that their good deeds deserve to be rewarded with thrones, and their sacrifices to be recompensed an hundred-fold. At this rate, all must be content to be debtors to God’s grace, and all talk of merit is out of the question. That is one reason why the rewards of the kingdom of heaven are so great. God bestows His gifts so as at once to glorify the Giver and to humble the receiver.
Consider how such rewards have the effect of creating the appropriate understanding of both disciple and Master. Human beings are very uncomfortable with the extraordinary weight of the Gospel – the sheer generosity of it – when rightly understood – overwhelms our pride. Consider the matter at some length: We have two minds when it comes to our standing before God. On one end we tend to consider it nothing, a matter we are owed and thus we take following after Christ lightly. Sin is nothing and holiness is nothing and forgiveness without cost – the “cheap grace” mentioned by Bonhoeffer. In fact, this problem appears in a form in the very next episode, where Jesus warns James and John of the cup to be drank.
In Matthew 10:38 (the first mention of the cross in the NT) Jesus lays out the bitter cost of discipleship; it will be everything:
32 So everyone who acknowledges me before men, I also will acknowledge before my Father who is in heaven, 33 but whoever denies me before men, I also will deny before my Father who is in heaven. 34 “Do not think that I have come to bring peace to the earth. I have not come to bring peace, but a sword. 35 For I have come to set a man against his father, and a daughter against her mother, and a daughter-in-law against her mother-in-law. 36 And a person’s enemies will be those of his own household. 37 Whoever loves father or mother more than me is not worthy of me, and whoever loves son or daughter more than me is not worthy of me. 38 And whoever does not take his cross and follow me is not worthy of me. 39 Whoever finds his life will lose it, and whoever loses his life for my sake will find it. Matthew 10:32–39 (ESV)
It is overweening pride which causes one to value the gift of God too lightly – and for these people, Jesus lays out the cost: everything. Yet, pride can cause us to fail in the other direction: We think and hope that we can pay something toward the gift of God. It is on this ground that much religion falls: the extraordinary penance of many throughout the world is a matter of extraordinary pride: but God will not sell. He will give bountifully, but he will not sell so that no one may boast:
1 What then shall we say was gained by Abraham, our forefather according to the flesh? 2 For if Abraham was justified by works, he has something to boast about, but not before God. 3 For what does the Scripture say? “Abraham believed God, and it was counted to him as righteousness.” 4 Now to the one who works, his wages are not counted as a gift but as his due. 5 And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness, Romans 4:1–5 (ESV)
Christ will cost us everything and then will reward us with gifts so great as to be beyond anything which we have lost. Indeed, when we see rightly, we will learn that we have lost nothing but our pride.