As this site summarizes her life:
Empress mother of Constantine the Great. She was a native of Bithynia, who married the then Roman general Constantius I Chlorus about 270. Constantine was born soon after, and in 293, Constantius was made Caesar, or junior emperor. He divorced Helena to marry co Emperor Maximian’s stepdaughter. Constantine became emperor in 312 after the fateful victory at Milvian Bridge, and Helena was named Augusta, or empress. She converted to Christianity and performed many acts of charity, including building churches in Rome and in the Holy Land. On a pilgrimage to the Holy Land, Helena discovered the True Cross. She is believed to have died in Nicomedia. Her porphyry sarcophagus is in the Vatican Museum. Geoffrey of Monmouth, England, started the legend that Helena was the daughter of the king of Colchester, a tradition no longer upheld. In liturgical art Helena is depicted as an empress, holding a cross.
At the same time, Helena was, and is, Waugh’s most intentional statement about the truth of Christianity, and about vocation – the divine call to a specific work in life – as the heart of Christian discipleship. Helena is full of biting historical and theological commentary (including a hilarious put-down of Edward Gibbon’s anti-Christian reading of Roman history). But, in the main, we are far, far away here from what one Waugh biographer calls the “jubilant malice” with which Waugh pilloried the California way of death in The Loved One. In Helena, Waugh explored, sparely but deeply, the question that shaped the last thirty-six years of his life – how does one become a saint?
In the course of his conversion to Catholicism, which took place in 1930, Evelyn Waugh came to the conviction that sanctity was not for the sanctuary only. Every Christian had to be a saint. And one of the hardest parts of that lifelong process of self-emptying and purification was to discover one’s vocation: that unique, singular something that would, in accord with God’s providential design, provide the means for sanctification. Helena’s sense of vocation, and the Christian scandal of particularity to which her vocation bore witness, was what attracted Waugh to the fourth-century Empress, whom the world remembers as the mother of the Emperor Constantine. Waugh later explained his choice in a letter to the poet John Betjeman, who confessed to being puzzled by the fact that, in the novel, Helena “doesn’t seem like a saint”:
"Saints are simply souls in heaven. Some people have been so sensationally holy in life that we know they went straight to heaven and so put them in the [liturgical] calendar. We all have to become saints before we get to heaven. That is what purgatory is for. And each individual has his own form of sanctity which he must achieve or perish. It is no good my saying, ‘I wish I were like Joan of Arc or St. John of the Cross.’ I can only be St. Evelyn Waugh – after God knows what experiences in purgatory.
"I liked Helena’s sanctity because it is in contrast to all that moderns think of as sanctity. She wasn’t thrown to the lions, she wasn’t a contemplative, she didn’t look like an El Greco. She just discovered what it was God has chosen for her to do and did it. And she snubbed Aldous Huxley with his perennial fog, by going straight to the essential physical historical fact of the redemption."
Waugh was not a proselytizer, and Helena is no more an exercise in conventional piety than Graham Greene’s The Power and the Glory, whose hero is an alcoholic priest. But Waugh was a committed Christian apologist, and his apologetic skills are amply displayed in Helena. Thus Helena was not only addressed to those Christians who were trying to figure out the meaning of their own discipleship; it was also intended as a full-bore confrontation with the false humanism that, for Waugh, was embodied by well-meaning but profoundly wrong-headed naturalistic-humanistic critics of the modern world like Aldous Huxley and George Orwell.
Read the rest of Weigel's comments here.