Is the promise 'I will never leave thee nor forsake thee' for believers today?
Yes. Though the promise was originally given to Joshua and Israel under the law, the book of Hebrews repeats it word for word, applying it to believers in the age of grace. Therefore, 'I will never leave thee nor forsake thee' is for believers today.
Les teaches that the Old Testament was addressed to the Jew under the law, but that does not prevent us from taking a statement like this and saying God says the same thing to me. He points out that the book of Hebrews practically repeats it word for word: 'I will never leave thee nor forsake thee.' Les explains that it is repeated to us as believers in the age of grace. So while all this back in the Old Testament was directed to Israel under the law, we can feed from it and learn from it, as Romans says. Therefore, the promise is for believers today.
Let your conversation be without covetousness; and be content with such things as ye have: for he hath said, I will never leave thee, nor forsake thee.