Sometimes we Buddhists get so offended when missionary teachings, especially Christianity and Islam try and convert other Buddhists. How much emphasis did the Buddha give to spreading the word of the Dhamma? Well, in one sense not too much. After he was enlightened, he thought "Enough teaching of the Dhamma, that even I found hard to understand, how much less will men dyed in lust and hate understand these truths? To teach would only be burdensome to me."
At that moment, Brahma Sahampati appeared before the Lord and requested him to teach, and then he was persuaded. Even with other Buddhas a similar pattern can be surmised. The Dhamma has often to be asked for before it is given. Those who are sincere and want to know something will find it. But, in the Cullavagga the Buddha encouraged his monks to walk and teach Dhamma "for the welfare and benefit of gods and men" and out of compassion for those with less dust in their eyes.
If in doubt, and generally based on my western Buddhist teachers, many of them proficient in Jhana the advice is "don't talk to people about this." The happiness in meditationn is sometimes so profound that it is best to keep quiet and let people ask first.
I find it so difficult to convert myself to Buddhism and this is what makes the teaching so vast, so challenging, so inspirational - to keep referring to it again and again. It would be far too much trouble to convert someone else given I can't even convince myself of certain meditation states.
Unlike some religions, Buddhism is not a club where when you have become a paid up member and been "baptized" you have a better afterlife whereas the unbelievers go to hell. Of course the Buddha did encourage and praise taking refuge in the triple gem as being of great benefit (Kutadanta Sutta of the Digha Nikaya), but in the Sarakani Vagga of the Samyutta Nikaya he makes it clear that you don't have to be a Buddhist, only a good person to be destined for heaven.
With even a little mastery of Dhamma one tends to become very attractive to others and they can become very curious - and in this way, eventually you may lead others to Dhamma. Leading by example rather than preaching.
Because the teaching is so deep and can't be talked about easily. Buddhism is therefore not a world religion in the same way as the other relgions. Given this spirit of liberality, it is a miracle that Buddhism continues to gain supporters without big buck missionaries ... how does this happen?
I don't know, but the feeling I have is, when you look after yourself really well you will look after others and the spirit of love and harmony that you generate will help increase the real Dhamma which is not fundamentally "Buddhist" but was taught by the Buddha 2400 years ago.
It is about pleasant feeling, spontaneous thoughts and wisdom.