St. John serves to contrast with the other male characters in the book. Compared with Mr. Rochester, he is as ice towards his love, Rosamond, because he knows that they are not suited for each other and would be unhappy together. This shows his far-sightedness. However, Mr. Rochester is fire, passionate towards Jane and determined to marry her at whatever cost just because he loved her without caring what the consequences were, even though he knew that he was already married to Bertha.
Also, Mr. Brocklehurst was cheap towards Lowood, only allowing the bare necessities to the girls there but spent much money on his family, dressing them up and deemed it proper. In comparison, St. John Rivers is a humble missionary, delighting in sacrifice for others, who did not think for himself, rather, the Church and his vocation thinks for him. He shows this way of thinking when he asks Jane to marry him and states that the marriage was only because he felt that she would be good as a missionary's wife unlike Rosamond and could do much good with him in India, and so would like her to marry him and follow him to India for missionary work.
Using St. John Rivers, Charlotte Bronte also manages to contrast the two marriages offered to Jane. Marriage with Rochester represents the abandonment of principle for the consummation of passion, but marriage to St. John would mean sacrificing passion for principle. When he invites her to come to India with him as a missionary, St. John offers Jane the chance to make a more meaningful contribution to society than she would as a housewife with Mr. Rochester, and so offering her a freedom: the freedom to act unreservedly on her principles.
He opens to Jane the possibility of exercising...