Without a clergy, how has the Bahá'í Faith become the second most widespread religion on the planet, or grown to seven million adherents, in less than 170 years?
Bahá'ís believe their faith "is summoning mankind to safety and love, to amity and peace; it hath raised up its tabernacle on the heights of the earth, and directeth its call to all the nations." Thus, it is easy for them to share their happiness with others. "Lift up your voices and sing out the song of the Kingdom. Spread far and wide the precepts and counsels of the loving Lord, so that this world will change into another world, and this darksome earth will be flooded with light, and the dead body of mankind will arise and live; so that every soul will ask for immortality, through the holy breaths of God." - Selections from the Writings of 'Abdu’l-Bahá, p. 2
Followers of Bahá'u'lláh may lack the special training of clergy--indeed, they are still learning how to build His grassroots elected administrative structure--but they are encouraged and empowered by His instructions: "Immerse yourselves in the ocean of My words, that ye may unravel its secrets, and discover all the pearls of wisdom that lie hid in its depths. Take heed that ye do not vacillate in your determination to embrace the truth of this Cause--a Cause through which the potentialities of the might of God have been revealed, and His sovereignty established. With faces beaming with joy, hasten ye unto Him. This is the changeless Faith of God, eternal in the past, eternal in the future. Let him that seeketh, attain it; and as to him that hath refused to seek it--verily, God is Self-Sufficient, above any need of His creatures." - Gleanings from the Writings of Bahá'u'lláh, p. 136
Believers endeavor to "live the life" and to be the change they want to see in the world. The teachings they follow do not vary from town to town or country to country, but are part of one unified set of laws that apply planet-wide. Their religious books are translated into more than 2000 languages, and Bahá'ís can be found in many thousands of places in the world. The advent of the Internet has helped increase access to what Bahá'u'lláh taught.
Although there is no formal caste of missionaries who leave their homes to move to other places in search of converts, Bahá'ís do move about the world in order to spread their faith. In most cases, these believers are self-supporting and expect to become a part of their adopted community. They do not preach or coerce belief, nor do they attack other traditions, but offer information if asked. In general, they hope that the example of their lives will be the best teacher.
'Abdu'l-Bahá, the son of Bahá'u'lláh and His Interpreter, gave this advice on how to teach: "O friends, consort with all the people of the world with joy and fragrance. If there be to you a word or essence whereof others than you are devoid, communicate it and show it forth in the language of affection and kindness: if it be received and be effective the object is attained, and if not leave it to him, and with regard to him deal not harshly but pray. The language of kindness is the lodestone of hearts and the food of the soul; it stands in the relation of ideas to words, and is as an horizon for the shining of the Sun of Wisdom and Knowledge." - A Traveller's Narrative, p. 43
I think that method would be the most effective way to share any message, religious or not!