Read James account "in context" which is what we are talking about. No one is saying that they only did one thing in their life. James is talking about them doing "one thing" to exhibit their faith. Why is that so hard for people to understand?
This is a one time event that James used to "describe Abraham's faith in action".
James 2:21-22 (NASB)
[SUP]21 [/SUP] Was not Abraham our father justified by works when he offered up Isaac his son on the altar?
[SUP]22 [/SUP] You see that faith was working with his works, and as a result of the works, faith was perfected;
Here is the very same thing - James showing the one time act of faith that Rahab did.
James 2:25 (NASB)
[SUP]25 [/SUP] In the same way, was not Rahab the harlot also justified by works when she received the messengers and sent them out by another way?
These were one time events in their life that showed they had faith no matter how people try to make into something else. It's plain as day but people are free to believe whatever they want.
It's the same for us when we hear the gospel message of Christ and the forgiveness of sins. We have a corresponding action to the message and we believe with our heart and confess that Jesus is Lord.
Abraham believed the word of God and was declared righteous 15 years before he put his son on the altar.