Answer by taking a quote from the following article:

Mary Mary are an American gospel duo composed of sisters Erica Atkins-Campbell and Trecina Atkins-Campbell. Formed in 1998, Mary Mary was launched into mainstream recognition following the release of their best-selling debut album, Thankful (2000), which contained the hit single "Shackles (Praise You)". Their followup album Incredible (2002), reached number one on the Top Gospel Albums chart. Their third album, Mary Mary (2005), which contains themes the public interpreted as a channel to the group's experience, contains the worldwide hits "Heaven", "The Real Party", and "Yesterday".

The Atkins sisters, born Erica Monique and Trecina Evette, grew up in a large family with seven siblings in Inglewood, California. Their mother, Thomasina Atkins, is an evangelist and choir director at the Church of God in Christ. Their father, Eddie A. Atkins, was also a member of the Church of God In Christ, and a youth counselor, before he died in 2013.  Due to their proximity in age, the two sisters grew up closer to one another than to any of their siblings: Darrell Atkins (born 1965, the only living boy- the second boy died as a child), Maliea Atkins (born 1967), Erica (born 1972), Tina (born 1974), Delisa Atkins-Brown (born 1977), Thomasina Atkins (born 1979), Alana Atkins-Jamison (born 1985), and Shanta Atkins (born 1986). Later, they would work creatively with over half of their siblings; Thomasina, Alana, Delisa and Shanta Atkins all sing background on Mary Mary's albums and during stage performances and Delisa was originally a member of the group but left before they started recording professionally due to pursuing college. Eventually, all eight Atkins children united on the Bobby Jones Gospel show on BET.  During this time, Erica and Trecina entered the world of church choirs, traveling gospel shows, and television productions. The sisters enrolled at El Camino College to study voice. There they encountered the division between rigorous academic study of music versus the popular musical world. "We had to study classical and sing arias, which was fine," Erica told Times, "but the teachers would tell us if we sang anything else it would damage our instrument." In 1995 the two toured with Michael Matthews' traveling gospel shows, Mama I'm Sorry and Sneaky. Each sister subsequently toured as a backup singer for a variety of R&B acts, including Brandy.

Where did the members of Mary Mary get their start in music?
During this time, Erica and Trecina entered the world of church choirs, traveling gospel shows, and television productions.