I also wonder why is there a slOka (# 18.67) immediately after this which states :
(summarising) " You should never disclose this to someone who is devoid of austerities, one who is not devoted, one who is adverse to spiritual advancement and one who is envious of Lord Krishna".
1.Why exclude someone who is not austere?
2.Cant devotion come after hearing the Bhagwad-GeetA?
3.What is meant by spiritual advancement? Everyone gangs up on Sravana whenever he even mentions the word "spiritual"
4.and lastly who is that person who can be envious of Lord Krishna (may be some asurA or a rAkshasA)?
5.Does BG mention anywhere about going to temples and why should Tagore write about not going to temples to bring out the message of BG?
I have numbered your questions so that I respond in reference.
This sloka defines pre-requisites by defining who have not met them. These prerequisites only makes sense when one has understood the message itself somewhat.
1. In translation into English like the word Tapas into the word Austere, we lose a lot of context and meaning. Tapas itself has been described somewhat in detail in earlier verses. In this context, it means someone who has discipline achieved by observance of Nithya Karmas for example.
2. The word Bhakthi has many related meanings based on context. In this context, the teachings cannot make sense to one that lacks Shraddha and some understanding of what Isvara is. Without this apriori knowledge, B.Gita can only confuse people.
For example, for one who has some rudimentary love for humanity this poem can somewhat make sense. For others it can come across as mockery of Hindu traditions. Suppose there are many more such poems, one will only get more turned off.
Similarly, Bhakthi is not going to grow if minimal pre-requisites are not met.
3. I do not read all the posts here and I am not into superstitions, magic etc. The word spiritual is abused and the word conveys different things to different people. I have not ganged up on Sri Sravana so I cannot relate to your comment.
'adverse to Spiritual advancement' in my view is poor translation to 'न चाशुश्रूषवे'
It is more about someone who is unable to listen and process information and such a person cannot relate to the message B.Gita.
4. In the entire B.Gita, Vyasa presents Sri Krishna as Bhagavan. There is never a line that says 'Sri Krishna uvacha'
For many Sri Krishna is portrayed as Mr Krishna who did magic. They may have no regard for his views because they cannot relate to him as Isvara. When Bhagavan says "No one is superior to Me" etc, it can rub one's ego the wrong way if they take Sri Krishna as another egoic person. For such people teaching will not make sense.
I have learnt from this forum that certain messages such as those in B.Gita or Upanishad cannot be received at all by some and the best is to take advice of Sri Krishna and shut up. My weakness is that I keep forgetting this, only to engage with people with no Shraddha and find once again that it was a wasted mental energy.
5. B.Gita does not mention about going to temples. I am not sure when temples appeared in ancient India. In Vedic culture the emphasis was more on agni based rituals. B. Gita does mention about simple act of worship (Pushpam, Phalam, Thoyam etc) .
Verse 9.26 describes this
पत्रं पुष्पं फलं तोयं यो मे भक्त्या प्रयच्छति ।
तदहं भक्त्युपहृतमश्नामि प्रयतात्मनः
There is not separate message of B.Gita vs those of other scriptures or by other teachers.
There is one message and many perspectives are offered.
What the poem is telling you is the essence of what it is to worship Isvara. It is not to say - do not go to temples, or church or synagogue or mosque. It says that in contrast to ignoring other things - like not being able to see the presence of Isvara of the beggar who is begging in front of the temple entrance.
Even in B.Gita, Sri Krishna does not start the teaching with acts of Bhakthi but about Sankhya (Knowledge) and then Karma Yoga etc.
They are all related concepts and that is why understanding what Bhakthi is requires an open mind.