To quote Rav Shimon Shkop's take on that gemara (Chagigah 15b; translation mine, taken from "Widen Your Tent: Thoughts on Life, Integrity & Joy", ch. 1, available here, pp 71-73):
But to my mind it is worth knowing and
contemplating what our Sages said on
Chagigah 15b. How could Rabbi Meir
receive Torah from the mouth of Acheir
[the former Elisha ben Avuya, after
he became a heretic]? Doesn’t Rabbah
bar bar Chana quote Rabbi Yochanan
[in Chagigah as saying], “What does it
mean when it says, ‘For the Kohen’s lips
should keep knowledge; they should see
Torah from his lips, for he is the angel
of Hashem, L-rd of Hosts” (Malachi
2:7)? If the rav is similar to an angel
of Hashem, L-rd of Hosts, seek Torah
from his mouth. And if not, do not seek
Torah from his mouth.” And the Talmud
concludes, “There is no question—this
[Rabbi Meir studying under Acheir] is in
regard to someone great, this [the verse]
is of someone of smaller stature.”
It is worth understanding according to
this how Rabbi Yochanan spoke without
elaboration, since he speaks only of the
smaller statured, not the greats. One
may say that we should be exacting in
that which Rabbi Yochanan said, “Seek
Torah from his mouth” and not “Learn
from him.” For in truth, one who learns
from his peer does not learn from the
mouth of the person who is teaching
him, but listens and weighs on the scales
of his mind, and then he understands the
concept. This is not learning “from the
mouth of” his teacher, but from the mind
of the teacher. “Torah from the mouth”
is only considered accepting the concepts
as he heard them, with no criticism. And
it was by this idea that Rabbi Yochanan
spoke about accepting Torah from the
mouth [i.e., uncritically] only if the rabbi
is similar to an angel of Hashem, L-rd of
Hosts.
And according to this, hinted to in
Rabbi Yochanan’s words is a distinction
between one who is of smaller stature
and one who is great. The person of
smaller stature will learn Torah from the
mouth, for he is unable to decide what
to draw near and what to keep away.
Whereas a person of great stature who
has the ability to decide [critically] does
not learn Torah “from [someone else’s]
mouth.”
Similarly, it’s appropriate to alert
anyone who contemplates the books of
Acharonim that they should not “learn
Torah from their mouths,” they shouldn’t
treat everything implied by what was said
as a fundamental principle before they
explore well their words.
Something like a hint of this idea can
be learned from what the Gemara says
in Bava Metzia, Chapter “HaSocheir es
haPo’alim” [85b]: Rabbi Chiya said, “I made it
so that the Torah would not be forgotten
from Israel.” It explains there that he
would plant flax, spread out nets [made
of that flax, thereby] hunt deer, made
parchment [of their hides], and write
[on them] Chumash texts. This hints
that whatever is in our power to prepare
from the beginning of the Torah, it
is incumbent on us to do ourselves,
according to the ability that was inherited
to us to explore and understand. And not
to rely [passively] on the words of the
Gedolim who preceded us.
I am not saying I personally would advocate shutting off critical thought when learning Torah. And I don't think that's Rav Shimon's intent either.
Rather, one should look for a rebbe who is so overwhelmingly ahead of you -- "like an angel" -- that trying to second-guess him seems silly. You might ask and challenge, but you have all expectation that your questions will be answered.