Question: If Abraham kept all the laws of the Torah and even the one that the Rabbis gave us later why didn’t wasn’t he circumcised until 100 years old?
Answer: You’re asking a really interesting question. There are a number of reasons why Abraham waited. One simple one is that a bris milah (circumcision) can only be done once. If he had done it earlier on his own, it would have been too late to do the mitzvah (commandment) properly when he was commanded.
The truth is, though, that this particular mitzvah doesn’t make sense to do on one’s own. I don’t claim to understand all mitzvos, but many of them have a clear purpose in and of themselves. We do a mitzvah when we don’t steal, but it is also a very good idea in terms of society’s property rights. Not marrying one’s relatives is another mitzvah, but it also serves a very important purpose in framing how a family functions. So Abraham would naturally want to do these mitzvos even before being commanded; they are the “best” thing to do!
The mitzvah of bris milah, though, in addition to other purposes, is also a “bris”, a covenant. This particular mitzvah creates a bond of love between Israel and our G-d. Now, that’s not the kind of thing you can do on your own. For instance, I could offer marriage to my wife, but we’re not married until she agrees and goes ahead with it. So too with bris milah – Abraham couldn’t make a unilateral covenant. He had to wait until he was invited.