During the Finnish war, when it became apparent that Sweden would not be able to defend Finland against the Russian Empire, he worked actively to make Finland adapt to the new situation and to promote Finland's interests in Russia. He opposed resistance. He participated actively in the preparations for the Diet of Porvoo which saw the establishment of the Evangelical Lutheran Church of Finland as a separate and independent church from the Church of Sweden. He also became chairman of a committee that drafted proposals for supreme governance for the Grand Duchy of Finland. Tengström's good relationship with Emperor Alexander I influenced the development of the Grand Duchy of Finland.