An interesting feature of chapter 15 is the fact that verse 2 is the only mention of the name of Abram's chief servant, Eliezer. This guy has a few important roles to play, but for the most part he does it rather anonymously. Some have suggested that his name, which means "God is help", has a certain suggestive symbolism to it. There are a few moments in the life of Abram that are considered by Christians to be prefiguring of events in the early Church, and in those stories, Abram represents God the Father and Isaac represents God the Son. So what about the Holy Spirit? Well, apparently the third person of the Trinity, who is never given a name in scripture, but is sometimes referred to as a "helper" (the word translated "comforter" in John 14:26 could also be translated "helper") is represented by Eliezer. I may get back to this in a later chapter.
One of the reasons that Eliezer is so important at this moment is that Abram has no child, and verses 2-4 are, translated into plainer, more modern words: Abram says, "Thanks for all the wealth you've given me, God, but I don't have any children. I guess when I die I'll leave it all to Eliezer." God says, "No, I'm going to give you a child that will actually be your blood descendant." Then in verse 5, God says that Abram's descendants are one day going to be so numerous, trying to count them would be like trying to count the stars. Some people interpret this as meaning that the Jews will be plentiful, some think it means all nations that trace descent from Abram (which would include the Ishmaelites, the Edomites, and a few others) and some interpret it to mean all followers of Judaism, Islam and Christianity. One thing it clearly doesn't mean is Abram's immediate descendants, as he had only about eight.
Last thing worth noting before I address the objections is verse 6, a verse that Christians consider a very significant one. Off and on, I've had to address the issue of whether Noah was a "righteous" man, and of course, the SAB points out others that are called as such. In Christianity, (and perhaps in Judaism to an extent, I'm not sure) there's a concept of self-righteousness vs. imputed righteousness. Everyone who tries to be righteous by their own merit has the former, and the Bible says "...our righteousnesses are as filthy rags..." (The SAB has many notes on this passage, but they ought to add the "Language" icon to it; the original Hebrew phrase is translated delicately, and literally means "used menstrual cloths". How's that for an image?) It's not that it's wrong to try to do good, but that deep down, we're trying to fool ourselves into believing that we're better than we really are. Now imputed righteousness, which is the kind I believe Noah had, is where a person, despite their failings, decides to trust in the Lord and accept that He is good. The epistle of James talks a lot about how real righteousness of this sort is intertwined with doing good things, but most of the Bible makes it clear that it comes first and foremost from a relationship with God. Abram had this relationship.
Okay, now to the rest of this chapter. Abram cuts up some animals and has a vision. What's it all about? This is an odd one, as this is not a sacrifice in the usual sense. Note that no altar is used and the animals are not burned up. My understanding of this chapter is that it's a cultural thing from Abram's time. When two people wanted to make a covenant with each other, they would take some animals, chop them in half like this, and the two parties of the agreement would walk down the middle. Probably like the sacrifices, the idea was that the goriness of the scene would be a reminder of the seriousness of the matter, and that if one should break the covenant, they might suffer a similar fate. Sure, it's a bit barbaric, but it's a symbolic gesture that Abram would understand. Also, note that there is no mention of Abram walking down the middle, only that God, in the form of "a smoking furnace, and a burning lamp" goes through the middle. This promise to Abram is a unilateral covenant with no conditions: Abram will be the father of a great nation, and that nation will inherit the land of Canaan. The fact that this was not fulfilled in Abram's own lifetime was mentioned a few chapters ago, and I addressed it there.
Then God tells Abram that the Egyptian captivity is going to happen before all this comes to pass, and that it partly has something to do with the Amorites. I thought I had mentioned that before, but I can't find it, so let me risk repeating and explain. Apparently, many of the nations in Canaan were immoral, and God was planning to wipe them out, but He was going to give them a few more years to clean up their act before He got rid of them and gave their land to the Israelites. He didn't just destroy these nations because he wanted to give something to Abram; He was using the Israelites as a tool to get things done.
So, how long was the captivity? 400 years or 430? Well, it depends on how you count it. Note that the wording in this chapter is different than in Exodus 12:40. I believe what's going on here is that the young nation of Israel went into Egypt during a famine, and stayed there 430 years. However, it wasn't until they had been there 30 years that they became slaves. Remember, the period of slavery didn't start immediately upon arrival; at first they were welcome guests treated with honor. How many generations was that? The SAB says seven, which contradicts the claim of "in the fourth generation" in verse 16, but what is that verse talking about? I believe it's not talking about the fourth generation from Abram, but the fourth generation from the beginning of their slavery. That would mean four generations after the death of the tribal patriarchs. So Kohath is the first generation, and Moses is the third, making the generation after Moses (and Moses was quite elderly when he led them out of Egypt) the generation God is talking about. Of course, generations aren't always well-defined. My oldest cousin is about 40 years old, but her father recently remarried and had a new child. Age-wise, my cousin is old enough to be her sibling's grandmother. So what is a generation? In this case, it's oddly enough about 100 years! There's something I think is more worth questioning than the number of generations.