The actual verse quoted in Romans 4:3 is Genesis 15:6, but in fact God's covenant with Abhaham comes in four stages.
- Gen 12:1-3. The covenant Announced. The word ‘covenant’ is not mentioned here but Peter in Acts 3:23 makes it clear that God’s initial promises to Abram were part of the covenant. The three part of the covenant, Land, Nation, Seed are made clear.
- Gen 15. The Covenant Transacted. The word ‘covenant’ is used in connection with Abraham for the first time, and a sacrifice is made. It is important to note that circumcision is not part of the covenant at this stage. The blessings promised to Abram are on the basis of his faith alone.
- Gen 17. The Covenant Instituted. He receives a new name, and the sign of the covenant, circumcision,
- Gen 23. The Covenant Confirmed. Abraham’s faith is proved by his actions (cf. Heb 11:17) and the promises repeated.
The Abrahamic Covenant, like the covenants with Adam (Gen 3:15), Noah and david, is a ‘covenant of promise’ (Eph 2:12). The word
promise is used quite frequently in the New Testament with reference to Abraham (Acts 7:5; Rom 4:12; 9:4-9; Gal 3:5-29; 4:28; Heb 6:13-20; 11:9, 13, 17). The promises are ‘in Christ’ (Gal 3:17 NKJV. cf. 2Cor 1:20) as well as ‘of Christ’ (Gal 3:16); that is, they refer to Christ and are for those who are His by faith. The ‘Seed of the woman,’ spoken of in Gen 3:15, is shown to be also the Seed of Abraham, but the blessing is to the whole world. The promises are entirely gracious. There was nothing in Abraham to make him worthy of being the recipient of these promises. He was not brought up in a household that worshipped Yahveh;
‘Your fathers, including Terah, the father of Abraham and the father of Nahor, dwelt on the other side of the river in old times: and they served other gods’ (Josh 24:2-3). It was not the piety of Abraham which commended him to God, but grace alone through God’s sovereign election. Nor could Abraham do anything to bring the promises about; he and Sarah might have been married for about fifty years before ever God spoke to him (cf. Gen 12:4), and they were doubtless already resigned to childlessness, but God was pleased to show him unmerited favour.
Abraham believed the promises of God concerning Isaac, and looked forward to the day of Christ (John 8:56), but how much he foresaw of His death and resurrection is not clear to me.
Isaac had the same promises as Abraham given to him, as did Jacob (Genesis 26:2-5; 28:13-15).
You can read an article I wrote on the Abrahamic covenant some years ago here:
The Covenants Part IV. The Abrahamic Covenant
If I were writing it today I would probably change and/or add a few things, but I think it may be helpful.