Does (be to bear) _sound natural to you or it does old-English? [closed]

"be" + to + the infinitive can be used to express future events in contemporary English, mainly something that is arranged for the near future or something you are obliged to do. Wikipedia calls it the "be + to construction".

In this case "be" is used as an auxiliary verb: see Merriam-Webster, specifically under "auxiliary verb": 4. "used with the infinitive with to to express futurity, arrangement in advance, or obligation". Merriam-Webster gives as an example: "I am to interview him today". It can also be used to express "future in the past" with a past-tense form of "be", e.g. "She was to become famous."

As mentioned, it commonly suggests a future event that is already arranged or something out of duty, rather than something vaguer or more distant. Wikipedia says it is normally used of the near future.

It perhaps isn't used as commonly as other versions of the future (English has many ways of expressing future including "I will go...", "I'll go", "I'm going to go...", present tense, or present continuous) but it's still valid ("I am to go..."/"I'm to go..."). It is sometimes suggested to be old-fashioned, but to me "I'm to bring him coffee" or "I'm to see her later" sounds normal, although slightly more formal than some ways of expressing obligation (e.g. "I got to" or "I've got to" are less formal/standard).

"Your wife Sarah is to bear you a son" has that sense of something already arranged (although Biblical English is often not very similar to everyday English, depending on the Bible translation). It sounds quite formal and portentious, although in spoken English you'd say "Sarah's" not "Sarah is" which adds to the formality.

As well as the Bible, this form of words also seems common in news headlines, as psmears mentions in a comment. Perhaps this is because of its brevity and because it emphasises that something is arranged to happen soon. It's possible to drop the copula in a headline, so "President to visit" rather than "President is to visit".