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"In Russia, sectarianism manifested itself in a reluctance to take advantage of the legal possibilities which the First Russian Revolution succeeded in wresting from tsarism despite its defeat. The members of the Parly who considered themselves "more revolutionary" than the Party opposed participation in the State Duma and work in the trade unions and mutual insurance societies. Instead of hard work among the masses they preferred to wait proudly aloof for a new revolutionary crisis.
In the beginning many of the Communist Parties formed in capitalist countries after the October Revolution committed errors of a sectarian nature. At that time Lenin called this Left-wing communism an infantile disorder. These errors manifested themselves in a refusal to work in the trade unions headed by reactionaries and opportunists, to participate in bourgeois parliaments, make compromises when necessary, and in general to employ flexible tactics.
Manifestations of sectarianism have to be combated also today. The essence of sectarianism consists in isolation from the masses, the failure to take advantage of the available opportunities for revolutionary work and an effort to evade the vital issues raised by life itself. Whereas revisionism seeks to reconcile the Parly to capitalism, sectarianism deprives the Party of its ties with the masses, without which it is impossible to wage a successful struggle against capitalism, ll is therefore impossible to strengthen the Party without fighting sectarianism, whatever its manifestations."
~FOML