Hello, Henrieta and please allow me to contribute with my two cents.
First of all, I think the usage of 'until' in the example you gave is incorrect. If I understand the sentence correctly 'as soon as' would be more appropriate.
The tenses appearing in the clauses of the example are determined by the relevant principles of the reported speech:
Christopher: 'I will help you as soon as I come back".
Now you report his words like this:
Christopher promised (Past Tense) he would help me as soon as he came back. (A direct consequence of the opening phrase in the Past Tense)
Well, that's in my humble, non-native speaker's opinion, of course.
A sentence with a similar meaning and still using 'until' would be
Christopher said he wouldn't be able to help me until he came back.
To more experienced users: please correct me if I'm wrong. It's not going to be a devastating blow to my pride. After all, most of us are still in the process of learning. Thank you.