Preaching to the choir
To (pointlessly) try to convince a person or group to accept an opinion that they already agree with.
Origin - the short version
'Preaching to the choir' originated in the USA in the 1970s. It is a variant of the earlier 'preaching to the converted', which dates from England in the late 1800s and has the same meaning.
Origin - the full story
'Preaching to the choir' (also sometimes spelled quire) is of US origin. It clearly refers to the pointlessness of a preacher attempting to convert those who, by their presence in church, have already demonstrated their faith. The first reference we can find is from 1973. Many other references date from soon after that, which points to the phrase being coined in that year; for example, this from The Lima News, Ohio, January 1973:
"He said he felt like the minister who was preaching to the choir. That is, to the people who always come to church, but not the ones who need it most."
The phrase may not be old but it does express the same idea as an earlier phrase - 'preaching to the converted', and is almost certainly a follow-on from that. This dates back around a century further and is first cited in a letter to The Times in November 1857:
It is an old saying that to preach to the converted is a useless office, and I may add that to preach to the unconvertible is a thankless office.
From 1867 onward, the expression appeared frequently in the writings of John Stuart Mill, who was a considerable force in the forming of English culture at the time, and it is from his use of it that the phrase became widely used.
The idea has also been expressed in another phrase that refers to an unnecessary act, that is, 'kicking at an open door'.
George Edward Bateman Saintsbury, in The Peace of Augustans, 1916, used both terms in one sentence:
"One may be said to be preaching to the converted and kicking at open doors in praising the four great novelists of the eighteenth century."