In 1905 he became Archbishop of Finland, in 1911 a member of the Holy Synod, in 1917 Archbishop of Vladimir, appointed metropolitan; then, until 1924, Metropolitan of Nizhnii-Novgorod. After the death of Patriarch Tikhon (8 April 1925), in the interim he succeeded as vicar in the place of the imprisoned Metropolitan Peter (Polianskii), locum tenens of the patriarchate. In 1934 he was elected Metropolitan of Moscow and Kolomna, and after the death of Metropolitan Peter (late 1936) he became, from 19 January 1937, locum tenens of the patriarchate. In 1943, on 4 September, he was received, together with the Metropolitans of Leningrad, Alexis, and of Kiev, Nicholas, in an audience with Stalin; on 8 September he was elected patriarch.
Sergius’s importance lies in the fact that, a man still of the old regime, he re-established relations as normal as possible with the Soviet government, also seeking to keep Russian subjects abroad in subjection. Famous is the encyclical of the Holy Synod inspired by him (16–29 July 1927), in which unconditional loyalty to the Soviet regime is demanded, and his astonishing declaration (March 1930), on the occasion of an interview, that in Russia there was no religious persecution. During the war, from 22 June 1941, he published 23 encyclical letters and organized collections of money, among other things for a tank column, for aeroplanes, and also for war-wounded, widows, orphans, etc.
Sergius is also known as a theologian and missionary writer. His principal theological work is The Orthodox Doctrine of Salvation (1st ed., Sergiev Posad 1895); as the subtitle notes, Essay of an Explanation of the Moral-Subjective Side of Salvation on the Basis of Holy Scripture and the Works of the Holy Fathers, it deals rather with the subjective side of Redemption, underrating the objective side, especially Christ’s vicarious satisfaction. He also wrote on salvation and faith according to the doctrine of the “Orthodox,” Catholic, and Protestant churches (Moscow 1913). On 7 September 1935 the patriarchate of Moscow (i.e., practically the Metropolitan Sergius) condemned the doctrine of Archpriest Sergius Bulgakov on Sophia or Divine Wisdom. His views on the primacy of the pope appear in an article of his (in the monthly review of the patriarchate of Moscow, February 1944) entitled: Has Christ a Vicar in the Church?; he gives a negative answer; he does however admit that the whole universal earthly Church might one day come to have a single supreme leader (it is implied: the Patriarch of Moscow, the Third Rome).