After graduation from the seminary, Demarest served at several
Reformed Church of America congregations:
Catskill, New York;
Flatbush, Ulster County, New York; New Brunswick, New Jersey; and
Hudson, New York. Next, he was pastor of the
Flatbush Reformed Church of Saugerties, serving from 1841 to 1843. He was the first pastor of the
Second Reformed Church of New Brunswick, starting in 1843 and serving until 1852. He was president of the General Synod of the Reformed Church in America for the 1858–1859 term. In 1865, a fourth professor position, the Professorship of Pastoral Theology and Sacred Rhetoric, was established at the
New Brunswick Theological Seminary. Demarest was the first elected to that professorship and was inaugurated on September 19, 1865, at the
First Reformed Church of New Brunswick. He served for thirty-three years. ==Works==