I was once told that one should learn every part of Torah at least once, even without complete understanding. Once you have seen it once in "Shamayim" you will be able to reveal the depths of everything. Does anyone know of a source for this? I think it was said in the name of the Chafetz Chaim.
The Shulhan Arukh HaRav (YD) Hilkhot Talmud Torah (2:13) writes explicitly that one should study the entire Torah, and even if he does not understand something, he will be privy to it in the future:
ואף על פי כן יש לאדם לעסוק בכל התורה גם בדברים שלא יוכל להבין ולעתיד לבא יזכה להבין ולהשיג כל התורה שעסק בה בעולם הזה ולא השיגה מקוצר דעתו
Nevertheless, a person should study the entire Torah, including things he cannot understand, and in the future he will merit to understand the entire Torah which he studied in this world, which he didn't understand to to his lack of intelligence.
שֶׁגַּם בְּנֵי אָדָם הַשּׁוֹמְעִים הַתּוֹרָה שֶׁל הַצַּדִּיק הָאֱמֶת וְאֵינָם מְבִינִים אוֹתָהּ, לֶעָתִיד בָּעוֹלָם הַבָּא יָבִינוּ. כִּי עִקָּר הָתּוֹרָה שֶׁלּוֹ הִיא לְהַנְּשָׁמוֹת שֶׁלָּנוּ
Also people who hear the Torah of the true zaddiq and do not understand it, will understand it in the future age. For his main Torah is for our souls...
While not speaking about comprehension, but rather remembering it, Shulchan Aruch Harav (Hilchos Talmud Torah 2:10), quoting Pri Eitz Chayim writes that one should learn the entire Torah once, even though he will forget it because he doesn't have enough time to review it, and in the World to Come they will remind him of everything he learned that he forgot because he didn't have time to review it.
This story happened with Rav Gustman zt'l who asked the Chofetz Chaim when he was a young boy if he should learn a complex gemarah with all its intricate details, that he won't completely understand. The Chofetz Chaim answered yes because of the above rule. See link below regarding the above story. It starts on page 34 in the sample pages titled A Question for the Chofetz Chaim.