The above quote is from Christianity Today's interesting article, "Gentiles in the Hands of a Genocidal God". That we should read the Old Testament from the point of view of Rahab (ie. a gentile) is an interesting (and even helpful) interpretation, but I do not find it completely convincing (in all cases). Yes, Christians (gentiles) were not led out of Egypt and the Old Testament is not written to us directly. But (and I think it's an important but) we have become the people of God through Christ. After all, what happened to Rahab after Jericho? Wouldn't Rahab also be implicated after joining Israel? Jericho was, you may recall, the very first city Israel came to after entering the land. She may still be a Gentile, but in quite a different way since she has joined with Israel."My proposal is that to read this story properly, as Gentiles, is to put ourselves in Rahab's place. Our origin lies not with the people who hear the command to kill, but with those who are to be killed. We belong with those who should be devoted to destruction because we offend against the holiness of God. And yet what has actually happened is that, like Rahab, we have received mercy through faith in the God of Israel"
To me it seems that Cary's description of the Christian's place within Israel is to make them another tree (as if, after Jericho, Rahab went her seperate way) which the Jewish messiah saves, but Paul is fairly clear that God has actually grafted Christians onto the Jewish tree (Rahab did stay with Israel). Cary mentions that Christians aren't part of Israel because they did not need to follow the whole law, but Paul (a Jew) also doesn't see the need to follow the whole law either, because of Christ.
Connected to this last point, Cary describes Israel as, "people of Israel, descended from Abraham, Isaac, and Jacob". However, John the Baptist (among others) would disagree with this assessment. Israel are the people who trust and obey God, first within the descendants of Abraham, Issac and Jacob, then elsewhere (e.g. Rahab, the rocks, believers in Christ).
Cary's idea is good, as far as it goes, but I think the application is not to "Gentile Christians", but both unbelieving Israel and Gentiles (everywhere) - Look at Rahab, God is merciful.
A short Bible story (about Jericho) to close seems prudent: Joshua arrives at Jericho and is awaiting instructions from God on how he should proceed. During the night the angel of the Lord comes to Israel's camp and he is stopped and asked, "Whose side are you on? Ours or our enemies?"
The angel's answer is telling: "Neither"