Sometimes critics complain that the Church misrepresents its size because the reported number of LDS members includes a large percentage of inactives (the implication is that inactives are not believers, a questionable assumption).
Anyway, the LDS Church recognizes that issue and is upfront about it. Don’t let critics get away with implying that the Church is less than forthright in its representation of church membership.
According to the National Council of Churches, The Church of Jesus Christ of Latter-day Saints is the second-fastest-growing church in the United States. However, despite its increasing numbers, the Church cautions against overemphasis on growth statistics. The Church makes no statistical comparisons with other churches and makes no claim to be the fastest-growing Christian denomination despite frequent news media comments to that effect. Such comparisons rarely take account of a multiplicity of complex factors, including activity rates and death rates, the methodology used in registering or counting members and what factors constitute membership. Growth rates also vary significantly across the world. Additionally, many other factors contribute to the strength of the Church, most especially the devotion and commitment of its members.