So far, I have been able to trace only two clear uses of the term 'sectarianism' in the work of J.V.Stalin. Although to my mind, his practice was non-sectarian, his writings do not dwell on this very much.  The points that I think Stalin makes on this matter, are as follows:

1)  The need for "flexibility"

In text one, he is in discussion with party officials on how to combat  illusions regarding nationalism, that  in 1923, many still had in the state of the USSR. When Stalin talks of the manner of work required, he talks of a need for 'flexibility'. Only by being 'flexible', can the cadre rally around themselves the "majority of the working people".
(2) But "flexibility" is not the same as having no principles. And the commmunists must find the dialectical balance between "strict adherence to principle" - and "sectarianism"

Yet it is not the case that 'flexibility' is 'opportunism' or an un-principled loss of "adherence to principle". There is a dialectical balance that must be found - between "strict adherence to principle" - and "sectarianism". This is taken from his discussion with the CPG member, Herzog, in 1925:

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