Lutheran IS Christian.
It's easy to think of Lutheranism as just another flavor of Christianity among many. But it's important to remember that the Christian faith, as confessed in the Lutheran Confessions, is simply Christianity in its purest form.
This is quite a claim I know. But there are no other Christian traditions that do not fall prey to the influence of Enthusiasm in one way or another.
Only in the Confessional Lutheran church is the true religion of Jesus Christ, as revealed to us in Holy Scripture, properly confessed.
It's often stated as a joke, that Lutherans believe that only Lutherans will be in heaven (inaccurate as that may be). But if the Holy Scriptures are correct, taken as a whole, with proper attention to context, and with the proper distinction made between Law and Gospel, the result is the pure Apostolic faith once delivered to the saints -- in common parlance, "Lutheranism." So it is correct to say that only Lutherans will be in heaven, because in heaven everyone will be Lutheran!
Here is a really good article that lays it all out very nicely:
Lutheran Exceptionalism - World Wide Wolfmueller