Teach that person carefully until he becomes a Baha'i.

Never must they let a day pass, without teaching some soul, hoping that Bahá’u’lláh will cause each seed to grow. The Friends should seek pure souls, gain their confidence and then teach that person carefully until he becomes a Bahá’í — and then nurture him until he becomes a firm and active supporter of the Faith.

http://reference.bahai.org/en/t/se/MC/mc-251.html