who is abraham's seed?

First read Genesis 12 and Genesis 13 to find the section where God is promising Abraham about his descendants.

Next read Galatians 3:15-21: When Abraham was promised the land to be inhabited by his descendants, there is a correction: Scripture doesn’t say “to his children,” as if it meant many descendants. Rather, it says “to his child”—and that, of course, means Christ.

Using the concordance link (left sidebar) read again the parts you found in Genesis, specifically the word descendants (Strong's number 2233).

So what appears to have occurred is the translation has caused the word seed to be read as a plural (more than one item) rather than the single (only one item).  This is similar to how we use the word sheep or fish. According to the correction made in Galatians, who is the seed?

Therefore, if the same word is being used in Genesis 15 and Genesis 22, then their meanings will need to be reexamined.

Perform the same concordance research and look for the sections that speak of God's promises when he uses the analogies of stars and sand.

Now to gain an understanding of what God was really promising Abraham:

1. Write out the following quote from Gen 22:17-18 "indeed I will greatly bless you, and I will greatly multiply your descendants as the stars of the heavens and as the sand which is on the seashore ; and your seed shall possess the gate of their enemies. In your seed all the nations of the earth shall be blessed, because you have obeyed My voice."
2. Replace the word descendants with the answer to who the seed is
3. Reading the sentence like this sounds a bit weird so you will need to discover how a few other Hewbrew words translate (highlighted)
4. Use the meaning which makes sense in the new context
5. Feel free to look at any other words you need to

When I did it, it came out as follows:

"Indeed I will greatly bless you, and I will make  Jesus mighty.  He will be as the stars of the heavens and the sand on the seashore.  He will possess the gate of the enemy.  In Jesus all the nations of the earth will be blessed, because My voice was obeyed."

Now read John 10:1-8 and John 11:52 from the new testament.  Remember to read the surrounding text for context.  Does anything sound familiar about God's promise?