Ron Sider points out what the impact could be made by evangelical Christians if they did something that most of the world doesn't consider either "astonishing" or "generous" at all, but just simple faithfulness: tithe.
...in 2002, just 9 percent of Barna's narrow class of evangelicals tithed...American Christians live in the richest nation on earth and enjoy an average household income of $42,409.The World Bank reports that 1.2 billion of the world's poorest people try to survive on just one dollar a day. At least one billion people have never heard the gospel...(but) if American Christians just tithed, they would have another $143 billion available to empower the poor and spread the gospel. Studies by the United Nations suggest that just an additional $70–$80 billion a year would be enough to provide access to essential services like basic health care and education for all the poor of the earth. If they did no more than tithe, American Christians would have the private dollars to foot this entire bill and still have $60–$70 billion more to do evangelism around the world.
Okay, so maybe simple faithfulness can result in something that IS astonishing.