I tried to find some worldwide membership statistics on those 2 Denominations/Congregations.
Not sure why those 2 just don't join together and they would be a more formidal challenge to the non-saturday sabbath keeping Christians, esp. the RCC.
They both appear to have started within the last 2 centuries.
Seventh-day Adventist Church - Wikipedia, the free encyclopedia
The
Seventh-day Adventist Church is a Protestant Christian denomination distinguished by its observance of Saturday, the original seventh day of the
Judeo-Christian week, as the
Sabbath, and by its emphasis on the imminent second coming (Advent) of Jesus Christ and was formally established in 1863.
The world church is governed by a General Conference, with smaller regions administered by divisions, union conferences and local conferences. It currently has a worldwide baptized membership of about
16.3 million people.
As of May 2007, it was
the twelfth-largest religious body in the world, and
the sixth-largest highly international religious body.
Messianic Judaism - Wikipedia, the free encyclopedia
Messianic Judaism is a
syncretic religious movement that arose in the 1960s and 70s.
It blends evangelical Christian theology with elements of religious Jewish practice and terminology. Messianic Judaism generally holds that Jesus is both the
Jewish Messiah and "God the Son" (one person of the Trinity), though some within the movement do not hold to Trinitarian beliefs.
With few exceptions, both the
Tanakh and the New Testament are believed to be authoritative and divinely inspired scripture.
From 2003 to 2007, the movement grew from 150 Messianic houses of worship in the United States to as many as 438, with over 100 in Israel and more worldwide; congregations are often affiliated with larger Messianic organizations or alliances.
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In 2008, the movement was reported to have between 6,000 and 15,000 members in Israel and 250,000 in the United States.