Ch 16: Abram and Sarai were not able to have children, though God promised children from Abram’s body (though not explicitly from Sarai’s). Abram listens to the voice of Sarai, who tells him to impregnate Hagar her maid, so Abram can have a son through her. This is akin to Adam eating the forbidden fruit when Eve gave it to her–a man, or anyone, is responsible to do what the Lord commands and are not to be swayed by alternative voice. Once Hagar was pregnant, Sarai despised Hagar. The angel of YHWH (preincarnate Jesus, 2nd person of Trinity) appears to Hagar to comfort her and promises that a great nation will arise from her son, though his descendants will be violent and in much conflict with their neighbors. This prophecy has been borne out to a ‘T’, as we can observe in the Middle East today. Abram named the son “Ishmael”. Its interesting to note that Ishmael was named before Abram’s name was changed (Sarai too).
Ch 17: At 99 years old, YHWH appears to Abram, changes his name to Abraham and promises him that within a year he will have a son (through Sarai), who is to now be called Sarah. This chapter is significant. God promises Canaan to Abraham forever. He makes a covenant with Abraham, the sign of which is male circumcision. Circumcision is in itself a sign of our lives belonging to God. Later in Galatians we find the Jews making it a requirement of salvation under the new covenant, which it is not, and it becomes emblematic of the Law, which we are no longer under as new covenant believers (Gal 3:25).
I think Abraham doesn’t quite believe God yet about a son through Ishmael and begs that God allow Ishmael to be the heir to the promises God has made. God says no, the covenant will be with Isaac, a child of (God’s) promise. Abraham is then circumcised at 99 and Ishmael at 13.
Ch 18: YHWH appears before Abraham in the form of three men (Father, Son and Holy Spirit) and speaks to him. Sarah and Abraham invite YHWH in for a meal, making 3 cakes and slaughtering a “choice” calf, like a sacrifice. They again promise Sarah she will have a son and she laughs, just like Abraham when he was told. Isaac means “he laughs”. The men (YHWH in bodily form) tell Abraham they are going to destroy Sodom. Abraham negotiates (intercedes) with God to preserve Sodom if 10 righteous men are found in the city. Abraham began with 50, then 45, 40, 30, 20 and finally 10. He didn’t feel bold enough to go lower than that.
This episode, if I were a Jew, I would find very hard to understand. Jews believe God is invisible and not a man, and here we have YHWH clearly appearing as a man, in fact as 3. And in the previous chapter the “messenger of YHWH” appeared to Hagar, speaking as God.