How to find files that contain one criterion but exclude a different criterion

Since the exit status of grep indicates whether or not it found a match, you should be able to test that directly as a find predicate (with the necessary negation, ! or -not) e.g.

find . -type f -name "*.c" \( -exec grep -q "ABC" {} \; ! -exec grep -q "123" {} \; \) -print 

-q makes grep exit silently on the first match - we don't need to hear from it because we let find print the filename.

Since you're already using GNU extensions:

find . -type f -size +2c -name "*.c" -exec grep -l --null ABC {} + | xargs -r0 grep -L 123 

If you want to do something else with those files:

find . -type f -size +2c -name "*.c" -exec grep -l --null ABC {} + | xargs -r0 grep -L --null 123 | xargs -r0 sh -c ' for file do something with "$file" done' sh {} + 

Or with zsh or bash:

find . -type f -size +2c -name "*.c" -exec grep -l --null ABC {} + | xargs -r0 grep -L --null 123 | while IFS= read -rd '' file; do something with "$file" done 

For complex constrains, I prefer using a programming language: Awk

find -name '*.c' \ -exec awk -v RS="\0" '/ABC/ && !/123/{print FILENAME}' {} \;