This may be a post where opening your Bibles and following along (very) carefully would be helpful. I am going to make a distinction between a covenant formally established with Abraham, the covenant of circumcision, and the covenant of grace which was promised to Abraham, yet not then established or formalized.
Here is a promise given to Abraham with all the families of the earth in its scope:
And I will bless those who bless you,
And the one who curses you I will curse.
And in you all the families of the earth will be blessed (Gen. 12:3; also see, Gal. 3:6-8).
Another, distinct promise was given here, with physical descendants in its scope:
The LORD appeared to Abram and said; To your descendants I will give this land.’ So he built an altar there to the LORD who had appeared to him (Gen. 12:7).
Notice, no covenant has been formalized at this point, but you can clearly see two distinct parties respectively involved in each promise. Moving on to Genesis 15, this becomes clearer:
‘Now look toward the heavens, and count the stars, if you are able to count them.’ And He said to him, ‘So shall your descendants be.’ Then he believed in the LORD; and He reckoned it to him as righteousness (Gen. 15:5, 6).
This is the first promise repeated. It is this promise Abraham believed and was thus justified. Moreover, this promise has reference to a particular party, namely, numerous descendants made up of “families of the earth.” This is an echo of the same promise made in Genesis 12, that all the families of the earth would be blessed (Gal. 3:6-8).
On that day the LORD made a covenant with Abram, saying, ‘To your descendants I have given this land, From the river of Egypt as far as the great river, the river Euphrates: the Kenite and the Kenizzite and the Kadmonite and the Hittite and the Perizzite and the Rephaim and the Amorite and the Canaanite and the Girgashite and the Jebusite (Gen. 15:18-20).’
This is the second promise confirmed by way of covenant. In this second promise, interests are different than in the first. In hindsight, this covenant promise pertains only to Abraham’s fleshly descendants or sojourners who were brought into the covenant of circumcision. This is NOT a promise to all the families of the earth. There are, therefore, two different parties (Israelites and families (or nations) of the earth) and two different blessings (spiritual blessing and earthly blessing) being spoken of so far. Follow me further…
God said further to Abraham, ‘Now as for you, you shall keep My covenant, you and your descendants after you throughout their generations. This is My covenant, which you shall keep, between Me and you and your descendants after you: every male among you shall be circumcised… thus shall My covenant be in your flesh for an everlasting covenant. But an uncircumcised male who is not circumcised in the flesh of his foreskin, that person shall be cut off from his people; he has broken My covenant (Gen 17:9, 10, 13b, 14).’
This is the same covenant given in Genesis 15. How so?
Because the possession of the land (promised in the Gen. 15 covenant) depends upon inclusion within the covenant of circumcision given here in ch. 17. If one is not an Israelite, the land cannot belong to them; and one cannot be an Israelite without circumcision (cf. “shall be cut off from his people”). Now, there are not only two different parties and sets of blessings being discussed here, but introduced here is a condition required of those who are to be included in the covenant, that is, circumcision.
The circumcised descendants of Abraham and “all the families of the earth” are two different parties, each taking advantage of different promises. The former takes the land by condition of circumcision, and the latter are blessed by way of faith and no conditions are given for this promise (Gal. 3:7, 29).
The covenant of circumcision was founded on law or obedience (circumcision); the covenant of grace is founded on a promise consisting of unilateral, unconditional adoption by grace through faith.
Therefore, Paul says, “For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise (Gal. 3:18).”
Ergo, the covenant of circumcision was a substantively different covenant than that of the covenant of grace, each being for distinct parties, offering distinct blessings, and each being founded on a diverse basis—either law or gospel respectively.
 Some have referred to faith as a condition which is not necessarily wrong, but the Particular Baptists, one of which was Philip Cary in the 17th century, avoided that language since something conditional usually refers to something earned purely upon merit. Faith, however, is supplied by God and is therefore a consequent necessity of inclusion within the covenant of grace rather than an antecedent condition required in order to enter the covenant of grace.