A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 15:1DIVINE ENCOURAGEMENT. (Gen. 15:1-21)
After these things--the conquest of the invading kings.
the word of the Lord--a phrase used, when connected with a vision, to denote a prophetic message.
Fear not, Abram--When the excitement of the enterprise was over, he had become a prey to despondency and terror at the probable revenge that might be meditated against him. To dispel his fear, he was favored with this gracious announcement. Having such a promise, how well did it become him (and all God's people who have the same promise) to dismiss fears, and cast all burdens on the Lord (Psa 27:3).
gen 15:2Lord God, what wilt thou give?--To his mind the declaration, "I am thy exceeding great reward" [Gen 15:1], had but one meaning, or was viewed but in one particular light, as bearing on the fulfilment of the promise, and he was still experiencing the sickness of hope deferred.
gen 15:3Eliezer of Damascus . . . one born in my house is mine heir--According to the usage of nomadic tribes, his chief confidential servant, would be heir to his possessions and honors. But this man could have become his son only by adoption; and how sadly would that have come short of the parental hopes he had been encouraged to entertain! His language betrayed a latent spirit of fretfulness or perhaps a temporary failure in the very virtue for which he is so renowned--and absolute submission to God's time, as well as way, of accomplishing His promise.
gen 15:4This shall not be thine heir--To the first part of his address no reply was given; but having renewed it in a spirit of more becoming submission, "whereby shall I know that I shall inherit it" [Gen 15:8], he was delighted by a most explicit promise of Canaan, which was immediately confirmed by a remarkable ceremony.
gen 15:9Take me an heifer, &c.--On occasions of great importance, when two or more parties join in a compact, they either observe precisely the same rites as Abram did, or, where they do not, they invoke the lamp as their witness. According to these ideas, which have been from time immemorial engraven on the minds of Eastern people, the Lord Himself condescended to enter into covenant with Abram. The patriarch did not pass between the sacrifice and the reason was that in this transaction he was bound to nothing. He asked a sign, and God was pleased to give him a sign, by which, according to Eastern ideas, He bound Himself. In like manner God has entered into covenant with us; and in the glory of the only-begotten Son, who passed through between God and us, all who believe have, like Abram, a sign or pledge in the gift of the Spirit, whereby they may know that they shall inherit the heavenly Canaan.