The word is "rationality", and we should be trying to spread it, because rationality (in ourselves and in others) is useful (to ourselves and others).

The proper way to spread it is to show others how rationality can benefit them, and assist them in their development as rationalists.

Don't think about people as divided into two groups: those who are rationalists and those who are not. Rather, think of them as practicing rationalists or potentially practicing rationalists.

There is not a single strategy for spreading the word; the number of techniques needed to help people to become practicing rationalists is almost as large as the number of people there are.

Just as it would make no sense to bury a duckling in the soil and pour water on it, or to throw bits of bread at a turnip seed, it may make no sense to attempt to intellectually nourish your grandparents by giving them a link to this blog, or to try to awaken the rationalist within your significant other by challenging them to a game of chess.

So, rationality enjoins us to spread the word, but, as with all things, it enjoins us to spread the word rationally. In essence, all preaching should be personalized.