Answer: According to Kabbalistic sources, people came from ancient Greece to Judea to study even in the time of the prophets, and they wrote about that.
The German philosopher Johannes Reuchlin, for example, who lived in the Middle Ages says that: “my teacher Pythagoras, the father of philosophy, did not learn from the Greek, but actually from the Jews.”
The Jews did not hide their knowledge from anyone, and Abraham initially wanted to tell about and to teach this knowledge all over Babylon, but the Babylonians opposed him. It all depends on people’s desires. Even today we see that currently people don’t want it.
The wisdom of Kabbalah had to be concealed for only 1,500 years, since the first century AD until the time of the ARI because humanity had to develop internally and to mature during that time. Then, starting from the ARI onward, it was possible to reveal the wisdom of Kabbalah to everyone. Baal Shem Tov started doing so, and today it is open for everyone.
From the Kabbalah Lesson in Russian 1/22/17