But is it wrong for a Christian to say "Inshallah"?
Couple of things.
I have little problem with using the word "allah" for God. I mean, what right do we English people have to derive the word for the deity from our own word for "good"? However, if it begins to refer to certain concepts of God in Islam, I would indeed press against it, much as I'd push against an Englishman worshiping Thor or a Greek worshiping Zeus. God isn't like that.
"God-willing" needs a revolution of meaning in Islamic as well as western Christian circles. Often it's a capitulation to the will of God, instead of a hearty embrace of His will. It reminds me of the diseased man coming up to Christ, "
If you want, you could heal me." Such a capitulation and despair that could mean. And yet he's placing himself in Christ's hands, hoping for what God can do.
When he heard, "
I'm willing", was that some kind of despair to him? No! He could've exploded with the gratefulness for God's willing healing of him. Who would say, "God willing" in this case? But that's the proper situation to say it, "This is so right: God's willing!"
"I'm willing" -- let's conceive of God saying this before we say, "God willing". We should be wary of tossing this phrase around easily. Maybe God sees things as unjust, things that He will correct on the Last Day. Should we say, "Inshallah" / "God willing" to those things only half-completed? No! God will fix things. We should look forward to His fixes -- because
that is all of God's willing. Some present event is just a small sliver of what God wills to bring to His people.
And it'll all be
made for our good, too. As crazy as it sounds, it's true: even bad things are for the good of God's people (Rom 8:28). Because we have eternity to look forward to, and this present, limited, temporary horror of sin is a brief blip that will fade into the horizon as we learn more and more about the God of all glory.