Derekh Hashem, Part Four, On Torah Study

The difference in the studies is according to preparation: However the study of one who purifies and sanctifies himself with his actions draws down influence commensurate to the measure that he prepares himself. So according to the measure that he increases preparedness, so does the preciousness of his study and its power increase. That is what we see with the earlier Sages. For the Torah would crown them with a great power and give them stature and preciousness that was not found in the later generations. [This was] because of their greater preparedness than the later ones. And they even said about Jonathan the son of Uziel (Sukkah 28a) that at the time that he was occupied with the Torah, any bird that would fly above him would be burned as a result of the power of the concentration of the Divine Presence that descended upon him through his study.