If you're going on the Wiki Page, you obviously don't mind spoilers. So with that being said: Meliodas disbanded the Seven Deadly Sins with the goal to become the Demon King. The remaining six pretty much said "Eff that," and stayed as a group with the goal to bring Meliodas back.
So he can break the curse on Elizabeth and himself.
That was the promise he made to her.
"break the curse" or shit
There's also Meliodas' emotions who still consires himself as the captain of the Seven Deadly Sins, so I'm sure that when he returns to his body he'll rejoin the sin's again.