Why do I get "integer expression required"?

I'm trying to have a small function that I then call.

To keep it simple I just want to validate that 12 is divisible by 2,3 and 4

The code is:

divisible_by () {
  under_test=12
  from=2
  to=4
  divisible=0
  for ((check=from; check<=to; check++)) {
    echo "check= $check"
    if [ $under_test % $check -ne 0 ]; then
      divisible=1
    fi  
  }
  return $divisible
}

x=divisible_by
if [ $x -eq 0 ]; then
  echo "true"
else
  echo "false"
fi

Currently I get

./5_divisible_by_1_to_10.sh: line 16: [: 
divisible_by: integer expression expected
false

I'm also finding it hard to pass the number in as a parameter but maybe it's related.