"I definitely have quit this group 100 times, probably last week," says Tina Campbell, 37, the older sister of the pair. "Being sisters is the best and the worst."
Erica, 35, acknowledges she has thoughts of a career as a solo star.
"See I don't think about quitting, I think about me by myself," Erica says, adding with a laugh: "I'm not going to deal with this foolishness for the rest of my life."
They admit being around each other 24/7 is annoying at times, but that sisterly bond is also the very reason why they've stayed intact.
"I mean, if you cannot be completely frank and blatant and who you are with your sister, then I mean, Jesus Christ, where can you be yourself?" asks Tina.
Since their debut in 2000, Mary Mary has consistently been on top of the gospel world. Their latest album, last year's "The Sound," became the group's fourth CD to hit No. 1 on Billboard's gospel albums chart (their Christmas CD peaked at No. 2).
The group has also had strong success on the R&B and pop charts - especially with their first song, the upbeat "Shackles (Praise You)," which became a top 40 hit. The duo's newest single, the Auto-Tune-tinged "God In Me," may match - or surpass - the latter song as it continues to climb the charts and gain radio airplay. To boost the song, the group's enlisted hit singer-songwriter Ne-Yo for a remix.
"God In Me" is a non-typical gospel song, with its references to flashy cars and designer clothes. That kind of talk almost made them bypass the song.
Then they connected with the song's deeper meaning.
"But when I started paying attention to what it was saying ... this is how, this is why, this is what has enabled me, I was like, 'You got me!"' Erica says.
"We try to make sure everything we put out there represents what we represent, that is true to us first - lyrically, creatively, sonically. We want it to be banging, on point. We want it to be respected across the board," Tina adds.
The duo says they're hoping to create a brand for Mary Mary. They've have a list of upcoming projects, including a book, a bath and body line, a line of jeans and a TV show; they also are featured judges on the BET series "Sunday Best," a gospel singing competition.
They also say they'll continue to push boundaries as gospel musicians.
"We don't limit ourselves to that market. (We're) faith-filled, love the Lord, always going to talk about Jesus because that's what we do and how we live ... but we want to remove the limits and just shoot for the stars," Tina says.
"The mission and purpose is to spread the message of the love of God," adds Erica. "Now under that you have the music business, and so we all have our aspirations, and some of us want more than others."