Rule: Singular nouns denoting groups such as professions, nationalities, religions, or political parties require a(n) after be or become.
Examples of errors:
(1) Being teacher requires a certain amount of dedication.
(2) I always enjoyed music so much that I decided to become teacher of music.
The more general form of Rule 1.2.2 is that any singular count noun in intensive relation with a preceding noun requires an indefinite or definite article. There are in fact a few verbs other than be and become, and many nouns other than those in the categories named in Rule 1.2.2. which can enter into this construction, for example:
John was/became/remained/stayed/felt/looked/seemed/sounded a doctor/an Englishman/a Catholic/a Republican/a fool/an intellectual/a nice boy/a good friend.
Feel, look, seem, and sound in this construction are more often accompanied by like in American English, however (e.g. feel like a doctor, look like a fool).
To say that John and the noun following the verb in the examples above are in intensive relation means that they are equivalent in some respect and that the second noun phrase is a subject complement. An intensive relation can obtain between subject and subject complement, as in the examples above, or between object and object complement, as in Mary considered John a doctor. An intensive relation can also exist in nonfinite clauses, even if the subject is not expressed, as in (1) and (2). In (1) the unexpressed subject of being is the indefinite someone, a person, etc.; in (2) the unexpressed subject of become is I.
The nouns in this construction may have definite or indefinite reference. If the complement noun has definite reference, a definite determiner is required, for example:
John is my doctor.
John is the Englishman I introduced you to.
John is that scholar I told you about.
If the nouns are plural and indefinite no article is used, for example:
John and Mary are doctors.
John and Mary are scholars.
John and Mary are Catholics.
We also have to specify "count nouns" in Rule 1.1.2, because with mass nouns an article is not always required, as in
Time is money.
That was good advice.