A judgment handed down on Wednesday of last week, in the Consistory Court, by the Diocesan Chancellor, the Worshipful Nigel Seed QC, granted a faculty to St Stephen's, Walbrook, to sell the painting Devout Men Taking the Body of St Stephen, by Benjamin West.
The picture has been bought by an anonymous foundation for $2.85 million (£1.88 million), and will be loaned to the Museum of Fine Art, Boston, the website Art History News reported last Friday.
Judge Seed said that the painting compromised the integrity of Sir Christopher Wren's design of the building, and that the painting had probably been introduced to the church in 1776 without a faculty.
The CBC was party opponent to the faculty. Its legal counsel and witnesses acted pro bono.
In a witness statement, Mrs Sloman said: "We understand the temptation for churches to sell off valuable works of art; but if such sales are given validity through success in even one or two instances, the parish churches of England could quickly be stripped of many of the treasures that make them unique."
The sale of the West painting would "have serious repercussions, and create an unfortunate precedent for any one of our 16,000 churches seeking funding for repairs, sending a message that the way is now open for them to dispose of the treasures they have inherited to the highest bidder", she said.
Speaking on Wednesday, Mrs Sloman said: "A lot of paintings were introduced in the 18th century without a faculty. Our concern is what is happening now, and in the future."
The CBC had done "a huge amount", she said, to help churches address financial shortfalls, such as persuading the Government to increase the Listed Places of Worship Grant Scheme from £12 million to £42 million. But it was important for PCCs to realise that they were "curators, not owners".
Mrs Sloman said that the CBC would decide at a residential meeting later this month whether to seek leave to appeal against the ruling.
On Art History News last Friday, the art historian Bendor Grosvenor said: "If all paintings in British churches were subjected to judgments on the nationality of the artist, the quality of the work, and the compatibility with the architecture, we would have almost nothing left."
The Priest-in-Charge of St Stephen's, Walbrook, the Ven. Peter Delaney, declined to comment.