I agree! I also was rather bothered by the term and the whole attitude of the piece. But you are correct.
It would come across better if he said to examine ourselves to see if we’re trusting Christ.
Now, I know that he is quoting the Scripture verse and it does say that, but considering that these says being in *the faith* equates more to being in a religious system or something we can put ourselves in to, there would be better ways of expressing it.