Luther’s brilliant insight is that God makes faith wait for the promised gift so that we would love Him above the gifts He gives. Like a healthy tree, the fruits of faith which are love and hope only come over time. Otherwise we would turn even God into an idol, loving gift over the Giver (James 1:17; Rom. 1:25).
AE 5:202-3: Reason and all the Epicureans argue that God seems to be a liar, because He promises in a kindly manner and puts forth good words but gives things that are evil. He gives Jacob a blessing and, on the other hand, allows it to be taken away. On the contrary, Esau, who has been cursed, remains in the house with his children, his wives, and his whole relationship, and governs everything just as previously he was head of the household and a priest of the church. Jacob goes into exile and abandons his blessing. This indeed is the administration of divine matters! The cornerstone must be rejected in order that we may learn to distinguish between divine and human government, and also that Jacob’s blessing has not been lost, but that this is only a trial, to see whether he is willing to cling to it firmly in faith and to wait. For this way Abraham is also ordered to sacrifice his son, as though he were about to lose him (Gen. 22:2). Soon, however, he recovers him with greater glory and profit (Gen. 22:12). Thus David, after being anointed to be king (1 Sam. 16:13), lives in exile for 10 years, is a servant of servants, and is king only in name and in accordance with the empty words which he heard from Samuel. Yet he holds out, and he perseveres in faith and expectation, until the outcome corresponds in richest abundance to the promise. This is the constant course of the church at all times, namely, that promises are made and that then those who believe the promises are treated in such a way that they are compelled to wait for things that are invisible, to believe what they do not see, and to hope for what does not appear. He who does not do this is not a Christian. For Christ Himself entered into His glory only by first descending into hell. When He is about to reign, He is crucified. When He is to be glorified, He is spit on. For He must suffer first and then at length be glorified. Moreover, God does this in order to test our hearts, whether we are willing to do without the promised blessings for a time. We shall not do without them forever. This is certain. And if God did not test us and postpone His promises, we would not be able to love Him wholeheartedly. For if He immediately gave everything He promises, we would not believe but would immerse ourselves in the blessings that are at hand and forget God. Accordingly, He allows the church to be afflicted and to suffer want in order that it may learn that it must live not only by bread but also by the Word (cf. Matt. 4:4), and in order that faith, hope, and the expectation of God’s help may be increased in the godly. For the Word is our life and salvation. If in this life the children of Adam can say and believe what is commonly stated, namely, that what is postponed is not taken away, why should we Christians not think and believe the same thing?