It is a common phrase in Pope Francis’s mouth, that metaphor invoking the image of a field hospital. And it is an apt metaphor, for the Church does minister to those in severe straights, gravely damaged by sin.
But is there any reason to suggest that this metaphor is complete? That it describes the entirety of what the Church is and does? No, of course not. Even keeping to the tenor of usage in which the metaphor originates, we could make use of many more descriptors.
The Church is a Field Army. Yes, this Church that Christ founded: the one of which He said “the gates of hell shall not stand against it”. That’s a metaphor Christ himself gave us. And it is a field army that attacks a city’s gates. This is (among other reasons) why we call the Church here on Earth “the Church Militant.” This Church goes out and takes new territory: With its missionaries it attacks Satan’s holdings to recover from Satan what really belongs to God. It claims new souls for God by preaching Christ crucified to pagans and atheists and unbelievers of all types.
The Church is a Medic on the field of battle. Even before you get a wounded soldier into a Field Hospital, you need a medic to collect him off the battlefield and into the ambulance. The Church reminds its own members of grave sins that they would rather think are perfectly fine behavior. These soldiers, though Christian, are dead to God because of those grave sins, and they need their condition to be brought to their attention before they can be readied for emergency surgery. They don't need a missionary, they need a medic.
The Church is a Regular Hospital. A field hospital only treats the emergencies that arise from being in the field. Once the emergency is stabilized, the soldier is moved to a regular hospital. Regular hospitals deal with lots of non-emergency care. They deal with systemic infections, with surgery you can schedule in advance, etc. The soldier who has begun the process of turning away from grave sin, but still needs much more care before he is healthy, doesn’t need “Field Hospital” treatment, he needs ongoing and intense medical treatment on a stable basis. The sinner who has accepted that his addiction or habits are sinful and recognizes his great need of amendment, and wishes to turn away from sin, has already achieved what the Field Hospital is there for. Now he needs another kind of care, the ongoing steady care of regular medicine.
The Church is a Rehabilitation Center. After a soldier has been wounded, and treated until the wound is no longer an ongoing threat, he still has to recover full function. He may be missing a leg, or may need massive therapy before using that leg again, before he can walk. The soldier who has repaired his basic relationship with God by contrition and forgiveness still needs to re-learn how to walk without falling down every step of the way. He needs a Rehabilitation Hospital.
The Church is a Teaching Hospital. The Field, Regular, and Rehabilitation hospitals all need staff. They need professional doctors and nurses capable of seeing and recognizing spiritual illness for what it is and how to treat it. The schools and colleges, seminaries and monasteries of the Church train many of our “spiritual health professionals” for their work.
The Church is a Nutrition Specialist. Just as a sick man needs the right medicine, a healthy man needs good solid regular food. Food that will provide him the nutrition he needs. The Church provides the Bread of Angels, the Food that keeps on giving, for one who feeds on this bread is never hungry. The Sacrament of sacraments is THE work, par excellence, of the Church.
The Church is an Olympic Gymnasium. In addition to the regular health professionals, the Church also trains up the super-athletes of each generation. She is home to the St. Pauls, those who “have run the good race, who have fought the good fight”. The star athlete may have been unhealthy at one time, and (if he falls to injury) may once again need a hospital, but by and large he needs other kinds of attention than medical care. He needs coaching, he needs whirlpools and rub-downs and so on (and a cheering section sometimes). The saints and the nearly-there holy people in this world don’t need a field hospital (or any hospital), they are not gravely, critically ill any more. They need coaching and holy friendship in support, while they display top-level holy living.
Too narrow a focus on the Church under only ONE of these roles will distort the Church’s whole. Yes, the Church needs to minister to those in dire moral straits. But she cannot do so as if all possible needs are “dire moral straits.” This would be treating those in need of rehabilitation as if they still need emergency surgery, when they don’t. It would be NOT treating those whose role is as a good, healthy foot-soldier ready for battle, leaving them to shift for themselves – which is a sure way to make sure that they become injured foot-soldiers, as Napoleon realized. Starve your regular foot-soldiers of the heavenly bread received in liturgically sound, reverent, well-sung Mass, and pretty soon you’ll have your soldiers in a rout by the enemy. So, providing this isn't just "fussy nonsense", not for those soldiers being made ready for regular battle. It is core function for what the Church is.