Van Gogh was certainly religious, although his beliefs did change with time. As the son of a minister, Van Gogh naturally had a Christian upbringing.
His father was a Protestant minister and belonged to a part of the church with a moderate, liberal perspective. Vincent initially wanted to follow in his father’s footsteps, but soon realised that they had strongly differing beliefs. Instead of being borne out of doctrine, Vincent thought that belief should be founded in human emotions, and primarily the emotions of people from the lower social classes (workers and farmers).
Vincent ultimately came to see nature and human history as jointly symbolising ‘God’. Not a defined god, but ‘something up there’ – something that can’t be named. Actually, an extremely modern stance for the time.