Before he was circumcised, he believed God when he saw the pillar of smoke and the pillar of fire pass between the parts. That is the moment his faith is counted as righteousness. And similarly, we have Jesus' death, burial, and resurrection, which is represented of all the components of that event.
Jeremiah 34:19
And those who have transgressed My covenant and have not fulfilled the terms of the covenant they made before Me, I will treat like the calf they cut in two in order to pass between its pieces.
Those who have transgressed, or go against, outside of, or without, thus not fulfilling the terms of the agreement... will have to assume the role of the calf cut in two, since they do not recognize the legitimacy of Jesus' position as savior.
This is the context of that reference that leads up to it:
Jeremiah 34
10 So all the officials and all the people who entered into this covenant agreed that they would free their menservants and maidservants and no longer hold them in bondage. They obeyed and released them, 11but later they changed their minds and took back the menservants and maidservants they had freed, and they forced them to become slaves again.
12Then the word of the LORD came to Jeremiah from the LORD, saying, 13“This is what the LORD, the God of Israel, says: I made a covenant with your forefathers when I brought them out of the land of Egypt, out of the house of slavery, saying: 14Every seventh year, each of you must free his Hebrew brother who has sold himself to you. He may serve you six years, but then you must let him go free. But your fathers did not listen or incline their ear.
15Recently you repented and did what pleased Me; each of you proclaimed freedom for his neighbor. You made a covenant before Me in the house that bears My Name. 16But now you have changed your minds and profaned My name. Each of you has taken back the menservants and maidservants whom you had set at liberty to go wherever they wanted, and you have again forced them to be your slaves.
17Therefore this is what the LORD says: You have not obeyed Me; you have not proclaimed freedom, each man for his brother and for his neighbor. So now I proclaim freedom for you, declares the LORD—freedom to fall by sword, by plague, and by famine! I will make you a horror to all the kingdoms of the earth.