What "parable"? You yourself referred to the "lukewarm", seems a synonym with "tepid"?
"Following His will half heartedly" doesn't only mean "wishy washy", as I noted. "Half heartedly" also means incompletely without the difficult and challenging role of intelligence and without the compassion, the "love" that Paul speaks of. To equate "adamant" and "hard line" with zeal is adopt the easy way, the tepid way that does not search for the good, the intelligent, the higher principle that demands discretion and discernment.  That is the "fervent" I am happy to seek and avoid the "warrior" and the "exclusionist" mentality as a plague.