He was the son of parish priest Ole Rein (1729–1792) and his wife Margretha Hansdatter Ross.
[4] In 1814 he was elected to the Norwegian Constituent Assembly at Eidsvoll Manor as a representative with the Bergen deputation, which was led by Wilhelm Frimann Koren Christie.
He favored the Independence Party (Selvstendighetspartiet), and held animosity towards the proposed Union between Sweden and Norway.
The Constituent Assembly elected Crown Prince Christian Frederick of Denmark as King of Norway.
Rein's anti-Swedish sentiments might have cost him the position of Bishop of Bergen, which became vacant in 1816.
In 1817 he co-founded the patriotic magazine Den norske Tilskuer together with Christian Magnus Falsen and Herman Foss.