Matthew Henry on Genesis 15:2-6

Though we must never complain of God, yet we have leave to complain to him; and to state all our grievances. It is ease to a burdened spirit, to open its case to a faithful and compassionate friend. Abram's complaint is, that he had no child; that he was never likely to have any; that the want of a son was so great a trouble to him, that it took away all his comfort. If we suppose that Abram looked no further than outward comfort, this complaint was to be blamed. But if we suppose that Abram herein had reference to the promised Seed, his desire was very commendable. Till we have evidence of our interest in Christ, we should not rest satisfied; what will all avail me, if I go Christless? If we continue instant in prayer, yet pray with humble submission to the Divine will, we shall not seek in vain. God gave Abram an express promise of a son. Christians may believe in God with respect to the common concerns of this life; but the faith by which they are justified, always has respect to the person and work of Christ. Abram believed in God as promising Christ; they believe in him as having raised him from the dead, Ro 4:24. Through faith in his blood they obtain forgiveness of sins.