If some teaching is not inmediately effective, but the results (as described in the suttas) are seen after a long, dedicated practice, should we discard that teaching?
I'm not sure that "immediately effective" is a good translation -- may be misleading.
The Pali word is akaliko i.e. "without time" which I'd guess might have a range of meanings ...
- "Timeless" -- i.e. not limited to a specific time ... maybe "eternally" true
- "Immediate" (not "immediately effective") -- i.e. "relevant to" or "applicable" or "practicable" now
- Continual or continuous
... as well as "without delay". When you translate you have to pick to pick one translation (e.g. "immediately effective"), which may not convey the whole range.
See also What is Sat-Dharma?
Also he translated -- and I don't mean to criticise, but I don't know why he did it -- as "immediately effective" something that's maybe two (separate) adjectives in the original: i.e. "immediate" and "effective".
For example, see here ...
Svākkhāto bhagavatā dhammo:
The Dhamma is well declared by the Bhagavā:
visible here and now, immediate,
inviting to come and see, effective,
paccattaṃ veditabbo viññūhī ti.
to be individually ascertained by the wise.
... where "immediate" and "effective" are two separate words (akāliko and opaneyyiko), though the dictionary also gives "immediate result" as one of the meanings of akāliko.
Also maybe "immediately effective" is correct -- if it's understand as, "successful practice has a present effect".
That might lead to a "No true Scotsman" argument (e.g. "If it's not immediately effective then you're not practising it correctly"), but anyway.
If a practicant is not able to perceive something described in the suttas inmediately, should we discard such suttas a priori?
For example I read The Radio Amateur's Handbook when I was child -- that book explains how to build various radios, and starts with the "basics" -- an explanation of electronic components and the behaviours of electromagnetic radiation. I think the lessons in the book were "true", but I didn't/couldn't understand most of it when I read it -- I understood a bit more when I read it the second time, and a bit more again the third time.
So I don't think it's sensible to "discard" lessons, which we're told are true and useful, just because we don't understand them immediately.
That said I do tend to classify lessons into "useful" and "not useful", and a lesson that I don't [yet] understand might be not [yet] useful for me -- not that it's false, just that I'm not yet sure how to use it.
So bits of Dhamma which I don't understand, I tend to put "on the shelf" -- perhaps I don't understand how to use it, but I don't want to discard it either.
Also I think that a lot of the doctrine is meant to be practised, practical. If it's like other "practices", it's something that people ("students") become better or more skilled at, as a result of practice. If you view it like that, then you might prefer not to say, "If I can't practice it immediately -- do it perfectly without effort, without practice, without mistakes -- then it isn't 'timeless' and I can discard it."
How can we know if some alleged result is not perceived due to my own current lack of experience, or if it's because the teaching is false?
I think we were supposed to use the four great references to determine whether something is the Buddha's doctrine.