1. He was born with a silver spoon in his mouth.

2. I don't actually believe that. I'm just trying to play Devil's advocate here.

3. My mother was the worst. She was basically the devil incarnate.

Do these sentences sound natural?

Question

1. He was born with a silver spoon in his mouth.

2. I don't actually believe that. I'm just trying to play Devil's advocate here.

3. My mother was the worst. She was basically the devil incarnate.

Do these sentences sound natural?

Answer

All are commonly used expressions that sound 100% natural in spoken English.

Unlike "God", "devil" is not capitalized.

The only thing unnatural is talking so harshly about one's mother.  Even bad people don't do that.

Answer

Fine!

Answer

Short answer: 
Those sentences are not natural in spoken English but could be used in writing.

Long answer:
1). You would read a phrase like this in books.

2)   I've used this one in writing in political chat rooms as "Just to play devil's advocate, why isn't....?".   By saying "play devil's advocate" it's implied that you are taking a position different to your beliefs; you don't need to say it.

3).  Using the term "the worst" is something that teenagers are known to say (see 1990s US movie Clueless, for context) but I don't even know if modern teenagers say it anymore.

To call someone the devil incarnate is very strong words and wouldn't be used unless it's in an exaggerated way e.g. "that cat is the devil incarnate".  It would be strange to hear someone describe their mother that way.

I hope that this is of some assistance.

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