What is the Abrahamic covenant, why did it pass through Isaac and not Ishmael, and is it still in force?

The Abrahamic covenant has two phases. The spiritual phase passes into the New Covenant in Christ and is still in force; the physical national phase of land and circumcision ceased entirely at the Cross. Ishmael was circumcised but was not chosen as the covenant bearer, which is a question of office, not of his personal salvation. Let me untangle the two things that are getting mixed together, because once you separate them the difficulty disappears. The covenant God made with Abraham runs from Genesis 12 through Genesis 22, and it has two distinct phases. The first is spiritual. It is announced in Genesis 12:3, "in thee shall all the nations of the earth be blessed," and St. Paul tells us in Galatians 3:6 to 8 exactly who that includes: the Jews and the Gentiles who would receive salvation under the redemptive work of Jesus Christ. This is the covenant undergirding all of Scripture's soteriology, and it is the spiritual side that transitions into the New Covenant. When Zachariah says God "has visited and wrought the redemption of his people" (Luke 1:68), that is this phase arriving. The second phase is physical and national. It is the land God swore to give Abraham's descendants (Genesis 15:15 to 21), and it was inaugurated by circumcision, first with Abraham in Genesis 17 and then with every male of the Jewish race (Exodus 4:22 to 26; 12:44 to 48; Leviticus 12:3; Acts 7:8). That phase was fulfilled. All the land God promised was in fact given to the Jews (Joshua 21:43 to 45; 1 Kings 8:56), and even partially returned after the Babylonian captivity (Nehemiah 9:7 to 8). And it ceased entirely when Christ inaugurated the New Covenant by His death and resurrection. Hebrews 8:7 to 13 and 10:16 to 18, 2 Corinthians 3:6 to 14, and Colossians 2:14 to 15 say so plainly. So when people ask whether the Abrahamic covenant is still in force, the honest answer is: the spiritual phase is, gloriously, in the Church; the national phase is finished and will not be restarted, and no modern political arrangement resurrects it. Now your question about Ishmael. You have put your finger on a real distinction. Circumcision was commanded for every male in Abraham's house, and Ishmael received it. But being circumcised and being the bearer of the covenant line are two different things. When God says "in Isaac shall thy seed be called" (Genesis 21:12, quoted by Paul in Romans 9:7), He is not damning Abraham's other sons. He is saying that Isaac was the son selected to be the official bearer of the covenant. That is an office, not a verdict on a soul. Ishmael had just as much opportunity to begin and end among the children of the promise as Isaac did, because membership among the children of the promise is by faith in the promise, not by works and not by bloodline. And the reverse holds too: being the covenant bearer did not guarantee Isaac membership for life. Paul's whole point in Romans 9:6 is that not all who are descended from Israel belong to Israel. That is why Paul can also say "not all are children of Abraham because they are his descendants." The Jews wanted to bind God to bless them on the basis of physical descent or of the work of circumcision. Paul takes them back to the womb of Rebecca, where neither Jacob nor Esau had done anything at all, good or bad, to show that God elects the bearer as He wishes. Election in that passage is about who carries the line, not about who is predestined to heaven or hell. As for why circumcision at all, and why so severe a sign: look at what Abraham had just done. He was a hundred, Sarah was ninety, no child in sight, and he weakened in faith on the very point the whole promise depended on, the seed. He took Hagar. Then he laughed in God's face at the promise. So God says, in effect, you doubted Me on the seed, so start cutting, and begin with yourself, and let it be a sign between Me and you so you remember what happened. And God has the larger picture in view the whole time, because Christ is cut, and He bleeds, and it gives life and it gives the seed, and through that we become children of God. One more thing worth knowing, because it governs how to read these promises. Every promise given to Abraham, Isaac and Jacob has a double fulfillment: one for national Israel and one for the Church. And except for the burial ground he bought for Sarah, Abraham never owned any land on earth. Hebrews 11:9 to 10 and 13 to 16 tell you why. He was looking for a country made by God. So when God says "to you and to your seed I will give this land," and Abraham never receives it, the promise reaches past Canaan to the new heavens and the new earth. Note also that Paul insists on the singular in Galatians 3:16: the seed is Christ, and in Him, Christians. And the Hebrew word olam, which we translate "forever," can mean an unending period (Exodus 15:18) or a long defined one (Exodus 12:14, 17, 24), so "everlasting covenant" in Genesis 17 does not by itself settle the question. Context does, and the context is the New Testament.