The Assyrian believers were part of the 230 who were taken hostage a year ago. They were captured last February after IS took over several towns and villages.
It's claimed millions of dollars were paid in ransom to the extremist group.
Younan Talia, of the Assyrian Democratic Organisation, confirmed the 40 remaining Christian captives were released early on Monday and were sent on their way to the north-eastern town of Tal Tamr.
He said the release came after mediation led by a top Assyrian priest in northern Syria.
Mr Talia said IS demanded a ransom of 18 million US dollars (£12.77 million) for the group. He said the figure was later lowered following negotiations but he did not know the final amount.
A Syrian Christian figure said the worldwide Assyrian community launched a campaign for the captives' release shortly after they were abducted. He said a bank account was opened in the northern Iraqi city of Irbil and donations began to flow in from around the world.
"We paid large amounts of money, millions of dollars, but not 18 million dollars," said the man. "We paid less than half the amount."