"grave sin" is correct and usable in written English.
You can use this phrase to refer to a very serious sin or offense. For example, "Murder is a grave sin that carries a serious punishment."
Homosexuality is a "grave sin", he added.
The church does the same with sin and grave sin.
Surely homosexuality remains a grave sin in the eyes of the Sydney church?
She called the archbishop's involvement with Mr. Marcoux "perhaps an indiscretion, perhaps a grave sin".
Pope Benedict XVI today branded the sexual abuse of children and young people a heinous crime and a grave sin.
Virtually every woman approached nearby agreed — including Nadezhda Kirillova, 70, who believes that abortion is a grave sin.
The destruction of fruit-giving trees, even those belonging to an enemy, is considered a grave sin in Judaism.
The archbishop added that as an excommunicated priest celebrating Mass, Mr. Bozek would commit "a most grave sin".
He called those actions a "grave sin that hurts the heart of the journalist and hurts others".
Suicide is a huge taboo in Egypt, a majority Muslim country where taking one's life is considered a grave sin.
The parents insisted that his "lifestyle" was a grave sin, incompatible with their deeply held religious beliefs.
