"Money and other political and economic means must serve, not rule," the pope said, adding that "in a seemingly paradoxical way, free and disinterested solidarity is the key to the smooth functioning of the global economy."
The pope also urged the world leaders, including U.S. President Barack Obama, to help broker an immediate cease-fire in Syria and bring warring sides to the negotiating table. The leaders were holding their annual summit at Lough Erne resort in County Fermanagh, Northern Ireland, June 17-18.
"Peace demands a far-sighted renunciation of certain claims in order to build together a more equitable and just peace," the pope wrote, adding that peace "is an essential prerequisite" for protecting human life and eradicating hunger.
Pope Francis' letter, released by the Vatican June 16, was written in response to a letter Cameron sent the pope June 5 outlining some of the priorities the British prime minister intended to push during his one-year term as president of the G-8. Cameron said he wants to emphasize openness in economies, governments and societies through the support of free trade, tackling tax evasion and encouraging greater transparency and accountability in government actions.
In his reply, Pope Francis said if the work of world leaders was to have any impact, all political and economic efforts and policies must be seen as the means, not the end, with the true goal being the protection of the human person and well-being of all humanity.
While freedom and creativity must be guaranteed for people and societies, nations also "must promote and guarantee their responsible exercise in solidarity, with particular attention to the poorest," the pope wrote.
As Pope Benedict XVI made clear, Pope Francis said, legal frameworks regulating economic activity and measures aimed at remedying the current global financial crisis "must be guided by the ethics of truth."
"Therefore, concern for the fundamental material and spiritual welfare of every human person is the starting-point for every political and economic solution and the ultimate measure of its effectiveness and its ethical validity," the pope wrote.
"Moreover, the goal of economics and politics is to serve humanity, beginning with the poorest and most vulnerable wherever they may be, even in their mothers' wombs," he wrote.
"Every economic and political theory or action must set about providing each inhabitant of the plant with the minimum wherewithal to live in dignity and freedom, with the possibility of supporting a family, educating children, praising God and developing one's own human potential," the pope said.
"In the absence of such a vision, all economic activity is meaningless," he wrote.
Solidarity is the key to a healthy global economy and proper solutions to today's serious crises will require "a courageous change of attitude" that puts economics and politics at the service of people, not vice versa, he said in his letter.