A sharp distinction is often made today between the Abrahamic covenant and the Mosaic covenant. Many Christians, particularly in American evangelicalism, believe that God's covenant with Abraham was "unilateral" or "unconditional," while the covenant made on Sinai was "bilateral" or "conditional". Hence, according to many, the Mosaic covenant (or at least many details of the Mosaic covenant) was temporary, but the Abrahamic covenant is eternal. The contemporary relevance of this distinction goes beyond the ivory tower to the sensitive and complex question of modern Israel's right to the land of promise.
The most common argument for this distinction is made by an appeal to Genesis 15, where, in response to Abram's question about how he can know that he will inherit the land (v. 8), the Lord alone walks through the pieces of a sacrifice (vv. 9-21). According to those who argue for the unconditionality of this covenant, the point of this strange passage is that God alone is responsible for the fulfillment of the land promise, and therefore the land is supposed to be the inalienable possession of Abram and his children. This is probably the most substantial theological reason for the widespread Evangelical support of the current state of Israel.
But there is a major problem with this argument. Actually, the real problem goes back behind this argument, to the language chosen for the premise. I doubt that the majority of the Christians who make this claim actually believe what they are saying when they call the Abrahamic covenant "unilateral" or "unconditional". For a covenant or promise to be unconditional means, quite simply, that the recipient is not subject to any conditions. In the case of Abraham and his children, if the promise that they would possess the land was truly unconditional, then they would be free to live any way they wanted, without reference to God or man, and the possession of the land would be their inalienable right. But read Deuteronomy 28-30, or Leviticus 26, and ask yourself if that is really the case.
So what do we make of Genesis 15 then? Well, reading it within the larger context of Genesis 12-17, I think we can say confidently that the point is not that the promise of the land is unconditional, but that it is assured. When Abram asks God how he can know that the promise will really come to pass, the Lord responds by making a binding agreement with him, holding himself accountable to his own word. In the act of passing through the sacrifice the Lord was declaring that he would make a way for the promise to be fulfilled, beyond the seemingly endless cycle of exile and return, so that Abraham's family would indeed have a homeland. The prophet Isaiah would speak centuries later about the Lord acting "by his own arm" for the salvation of his people, and here at the beginning of his dealings with them we can see, however cryptically, the foundation of that divine commitment.
As Christians, we believe that this way was ultimately made open by the inauguration of the New Covenant through Jesus the Messiah, the forgiveness of sins and the giving of YHWH's own Spirit (e.g. Rom 3:21-26). But again, saying that the Lord would make a way, and hence that the promise is assured, is not the same thing as saying that the terms of the covenant were unconditional for Abraham or his children. As we see earlier in the same chapter, Abraham had to believe God's promise for it to be "accounted to him as righteousness," and after the compromise with Hagar in the next chapter the Lord appears to him once more with a warning that he must walk before him "blameless" if he is to inherit the promise (17:1-2).
So while it was ultimately the Lord's faithfulness alone that made a way for Abraham's family to meet the conditions of the covenant—through the death and resurrection of his own son, by the forgiveness of sins and the bestowal of his own Spirit—this does not change the fact that the promise was conditional, both for Abraham and thus for all who would likewise become its heirs. The only condition, as Genesis itself indicates and as Paul spells out in detail, is to "walk in the steps of faith which Abraham walked". Hence Paul's point in Romans 4: the "seed," the true descendants of Abraham, are those who respond to the promise in faith, by the Spirit, through the Messiah, and not those who merely descend from Abraham according to the flesh.