"Wrath of God" -VS- "Chord of Calling" + "Selfless Spirit" sacrificed in response
You are correct, you can save your creatures with that play.

The stack works according to the "last in, first out" principle. Whatever spell or ability goes on the stack becomes the top-most object on it, and when the stack starts resolving, the top-most object on the stack will resolve first. 

Whenever something resolves, players have the chance to play more spells and abilities. So the line of play goes like this:


Your opponent plays Wrath of God
You play Chord of Calling in response
Both of you pass on playing things, and the top-most object, Chord of Calling, resolves. You get the Selfless Spirit on the battlefield.
You sacrifice Selfless Spirit. Its ability becomes the top-most object on the stack. Wrath of God is still on the stack, at the bottom of the stack.
Selfless Spirit's ability resolves, making all of your creatures indestructible. 
Wrath of God resolves, and will not kill any of your creatures.