'earnest devotion' is a correct and usable phrase in written English.
For example, you could write, "Though John was not the most talented athlete, his earnest devotion to the sport made him a team favorite."
One couldn't help but be impressed by the actors' earnest devotion.
Their noxious attitude was seductive, and so, too, was their earnest devotion to the old-fashioned craft of hip-hop: subtle rhythms and unexpected rhyme endings, do-it-yourself beatmaking and engrossing storytelling.
Perhaps they are, but insisting on the benevolence of the universe in the way that "Life of Pi" does can feel more like a result of delusion or deceit than of earnest devotion.
Such earnest devotion to traditional art and craft was rejected by many of the people behind punk newspapers and zines — although there were plenty of artists, illustrators and designers working under the movement's umbrella.
In the first song we encounter Adriana flushed with romantic confusion and fear; in the second, the young woman anxiously wonders whether the child she is bearing will be shaped by the blood of his violent father or by her own earnest devotion.
"They were very earnest in their devotion to the United States of America.
If the inquirer can find a person to experiment upon who is well trained in a certain type of scientific work, intelligent and earnest in his devotion to it, and unaccustomed to metaphysics, let him probe into various presuppositions that his 'subject' has been taught to make in the course of his scientific education, and invite him to justify each or alternatively to abandon it.
"The ghastliness of this damp and squishy comedy is the byproduct of a confused and earnest sentimentality, a willful devotion to wide-eyed wonder that confuses simplicity with simple-mindedness".
They too have earnest songs about love and devotion, delivered with folky strumming and hearty vocal harmonies, but with more of a 1960s flavor than Mumford.
This devotion is sincere.
And while there is something wishful and secondhand about his devotion to them, there is also something earnest and touching.
