it = that. The sin unto death was one that might result in the brother being cut off. Compare 1Co_11:30, where many had sinned unto death "many sleep". See also Jam_5:14, Jam_5:16, where there is the same recognition of sickness being due to some special sins, as in 1Co_11:30, and of intercessory prayer as here. It is not a single act, but a continued habit.