I wonder if "angels of mercy" by itself (not in context) will be understood to mean "nurses."
I have googled this term, but the results I got referred to the angels from religions.
The association of nurses with "angels of mercy" goes back more than 175 years, and in at least some early accounts may involve the overtly religious inspiration of the nurses. Lucy Hamilton, "The Sister of Charity," in the [Clearfield, Pennsylvania] Democratic Banner (July 9, 1852) tells a story that seems likely to have historical underpinnings, especially with regard to the nursing staff at a hospital in Boston:
I chanced while making a tour of the Northern States, during the year 1820, to be taken suddenly ill in the city of Boston, and, in consequence, had myself removed from my hotel to the sick hospital, Sisters of Charity were among the regular attendants, numbers having volunteered their assistance on account of the difficulty of procuring nurses.
In the paragraph immediately before the one just cited, the writer gives a sense of the support that these nurses provided:
Reader, if, like me, you can recall the time when religious consolations breathed into your ear tenderly and spiritually, first penetrated a heart petrified by long sufferings, you know how to value those angels of mercy and kindness, who hover around the sick and dying, to minister support and sympathy in the dark hour of mortal despair.
Twenty years earlier, an item headed "Brothers of Charity" in the [Lawrenceburg, Indiana] Western Statesman (November 16, 1832) describes similar ministrations provided by male volunteers:
Brothers of Charity.—There is a society in this city, which for distinction's sake, we will call the Brothers of Charity. It is a part of their creed to hold themselves in readiness for every good work. Many of them are wealthy, and being so, possess the means of far more extensive usefulness than if otherwise circumstanced. Thy do not coop themselves up in solitary establishments, but mingle in society like other men—assume its relations, and perform its duties. Whenever distress befalls the community, you will be sure to find them, like angels of mercy, administering relief to the suffering. During the prevalence of the late epidemic, no place inhabited by human beings, was so dark and dreary, that they did not penetrate it; no atmosphere so loaded with pestilence that they did not breath it; no wretchedness so revoking that they did not relieve it. The sick and the dying were soothed by their kindness, the abodes of pollution were cleansed by their instrumentality, and at their expense; whatever humanity could suggest, was cheerfully rendered, at the sacrifice of every personal comfort; and all this was done day after day, week after week, by men who had thousands and tens of thousands at their command, and might, instead, have been rolling in the magnificence of earthly splendor, with no sight or sound of wo[e] to annoy them. ... The Brothers of Charity have no peculiar organization; they are simply good men and good citizens, carrying their principles into practice, and fulfilling their obligations, to themselves, their families, the community, and their Maker, in all the relations in which Providence has placed them.
And from "Sisters of Charity" in The Catholic Telegraph (May 25, 1833):
These Angels of Mercy, hearing that the city authorities of Philadelphia were about to present each of them with a piece of plate, with appropriate inscriptions, for their labors in the Cholera Hospitals, during the spread of the pestilence, have addressed a letter to the Mayor, informing him that they are unwilling to receive any guerdon for the perils which they voluntarily braved. They remark, in their letter, that they are 'aware that the offering was not to be presented as a recompense for their services, but as a mark of public approbation of their conduct. If their exertions have been useful to their suffering fellow beings, and satisfactory to the public authorities, they deem it a satisfactory reward, and indeed the only one which it would be consistent with their vocation to receive.' Here we perceive that mercy, which droppeth like the gentle dew from Heaven upon the Earth beneath.
Medical nursing seems not to have been viewed as a profession prior to 1850 or so. Rather, it was a do-gooder volunteer activity performed by relatives, the saintly, or (in times of war) the ultra-patriotic. This born out by the first paragraph of Encyclopædia Britannica's discussion of the history of nursing:
History of Nursing
Although the origins of nursing predate the mid-19th century, the history of professional nursing traditionally begins with Florence Nightingale. Nightingale, the well-educated daughter of wealthy British parents, defied social conventions and decided to become a nurse. The nursing of strangers, either in hospitals or in their homes, was not then seen as a respectable career for well-bred ladies, who, if they wished to nurse, were expected to do so only for sick family and intimate friends. In a radical departure from these views, Nightingale believed that well-educated women, using scientific principles and informed education about healthy lifestyles, could dramatically improve the care of sick patients. Moreover, she believed that nursing provided an ideal independent calling full of intellectual and social freedom for women, who at that time had few other career options.
But if Florence Nightingale—the lady with a lamp—marks the origin of the professional nurse, she carries the "angel of mercy" description into the new profession with her, as this illustration titled "Florence Nightingale, an Angel of Mercy. Scutari Hospital 1855" indicates.
To some extent, whether people will recognize the phrase "angels of mercy" as an allusion to nurses depends on the context in which it appears. But the phrase has been applied to nurses since the days when nursing was often viewed as a kind of ministry to the sick that might or might not include any training or scientific competence. The connection is thus firm and of long duration.