It is left to the Rambam to fill us in. In the Laws of Avoda Zarah(1:3), the Rambam informs us that Avraham recognized God at the age of forty. This information, however, is quite peculiar in light of the fact that the Rambam’s Mishneh Torah is a practical work. Why does he need to tell us this information?
Avraham was not the only one to discover God at the age of forty. In Avos D’ Rebbe Nosson (1:6) we are told that Rebbe Akiva began to study Torah at the age of forty. Was it merely a coincidence that they both changed their lives around at the same age? Was it an ancient form of a mid-life crisis?
Observing a hole in a rock created by dripping water inspired Rebbe Akiva to realize that Torah could penetrate his heart. Similarly, Avraham was stirred by what he saw in everyday occurrences. Bereishis Rabbah (39:1) uses a parable of someone who upon witnessing a burning building realizes it must have had a builder to describe how Avraham, upon looking at the world realized it must have had a creator.
It seems that the age of forty was not a coincidence at all. The Mishnah in Avos (5:21) points out that at the age of forty one reaches the stage of Binah. Rashi in several places writes that Binah is the ability to understand one thing from another; to gain insight from something one already knows. The age of forty allowed Avraham and Rebbe Akiva to look at everyday events and see what was obvious; that there was a creator.
Parshas Vayera contains a phrase that appears nine times in chumash, three times. “Vayisa es einav vayar”- “He raised his eyes and saw”. From the beginning of the parsha to the end Avraham was looking. He looked at things we take for granted and saw miracles. This is how he discovered God.
Countless things in nature and everyday occurrences are testimony to God’s existence. All we need to do is look.