The unprecedented legacy is believed to be in the region of £750,000, although the exact figure has not been confirmed.
A statement from BMS board members the Rev David Kerrigan and Jeff Taylor said the sum was "a wonderful answer to prayer".
They acknowledged, however, that the legacy is a temporary fix to an otherwise uncertain time for the organisation, which specialises in holistic mission in more than 40 countries around the world.
"Although a legacy is a one-off gift and not a source of regular income, it allows time to consider whether new options become available over the coming year," the statement read.
"Importantly, it also enables us to step back from what previously seemed an unavoidable decision to make a number of posts redundant."
The board added that it would formally review the position in the summer of 2009.
BMS originally planned to lay off staff at its headquarters in Didcot and a number of area coordinators as it has struggled to break an average annual deficit of £300,000 for the last three years. Just one week after the announcement of staff cutbacks, the organisation received news that it was one of three Christian charities to benefit from a sizeable estate, according to Baptist Times.
Mr Kerrigan told the newspaper: "A one-off gift does not overcome the problem that we are still spending £300,000 more than our income.
"But it helps us to step back and find other ways in which we can close the gap."
Mark Craig, BMS Director for Communications, added that the organisation remained hopeful that its mission work could continue in spite of uncertainties. Already this week, new mission workers arrived in Africa and Latin America, he said.
"This is what BMS is about, and we are determined to resolve our financial position, to allow us to refocus all our efforts on our mission work."
