noun
1 sb who has religious faith
ADJECTIVE
▪ devout , genuine , true
▪ religious
▪ Christian , Muslim , etc.
▪ fellow
▪ new
▪ ordinary ( esp. AmE )
2 sb who believes that sth is good
ADJECTIVE
▪ avid ( esp. AmE ), fervent , firm , great , passionate , staunch , strong
▪ big , huge ( both informal )
PREPOSITION
▪ ~ in
▪
I'm a firm ~ in the benefits of exercise.