I think that one of the most profound impacts of Modernism from a cultural or spiritual point of view was its embrace of a lack of totality in the world. The alienation that became part of the Modernist movement during World War I ended up displacing the belief in a God or divinity. Whereas individuals used to take solace in religious spirituality or in the collective sense of social solidarity, these elements became dislodged as a result of Modernism during World War I. The entrance into "the Great War" was heralded as an affirmation of old world notions of the good. National allegiance was a motivation for entering the war. Social solidarity was cited as a rationale. Even spiritual belief was used as a reason to enter the war. As the conflict progressed and more people became its victims and even more started witnessing the sheer "horror of war," the alienation that was a part of Modernism settled into the psyches and conditions of individuals in an even more profound manner, almost as if the war was a confirmation that Modernism was right. When Woolf says, "All human relations shifted," she might have been speaking of the change that Modernism had on how people viewed cultural and spiritual notions of the good.
WWI did not just shatter belief in God. It also shattered the Enlightenment belief in the perfectibility of humankind. After WWI, people no longer believed that the world was inevitably (if slowly) moving towards greater civilization. This brought to the fore the sort of doubt and uncertainty that characterize modernist thought.
WWI, then, broke down the certainties that had been in place since the Enlightenment and created a culture that had less faith in either God or man.