The Sun Also Rises is a quintessentially modernist work in many respects:
1 - its charcters are more concerned with their own moral code and they pursue their own goals rathern than trying to fulfil social expectations and to fit into the larger society (as a character in a Victorian novel would do). This has led them to become expatriates, to leave their country of origin to be freer. They are also deeply disillusioned and do not practice religious beliefs. Jake actually defines himself as a "rotten Catholic";
2 - as several modernist works, it is haunted by the memory of the First World War as a traumatic event. The scar of the war is visible on Jake's body whose mutilation has left him sexually impotent;
3 - it challenges traditional gender roles and shows strong female characters like Lady Brett Ashley. She is assertive and adheres to a liberated sexual code. This may seem obvious to today's readers. Yet, not many female characters before Brett had expressed their sexual desires so freely;
4 - stylistically, the first-person narrative seen through Jake's eyes, the intense use of dialogues to further the narrative and the short, brief sentences that give the narrative an almost journalistic quality are all modernist devices.