The recorded conversation makes it clear what the followers of Venat thought.
Venat: Yet it is plain they will not countenance a permanent solution. That being the case, we must ask ourselves a simple question: are we prepared to pursue our chosen course, even should it mean suffering the eternal condemnation of our brethren?
If so, I see no further reason to demur.
Let us bring forth the Light that shall ever after keep the Darkness in check.
Urianger himself says it plainly right after the recording ends.
Urianger: For a certainty, Emet-Selch never uttered the name, speak though he did of the schism which Zodiark's advent wrought upon mankind.
Some were of the viewpoint that His power must needs be restrained, and to that end did they call forth She who would serve as His shackles.
Now, let it be clear I'm not saying that it's impossible for their to be other reasons and motivations, but as it stands this is the reason that's stressed in the dialogue so far.
And it makes sense. By this point Zodiark has halted the Final Days, and repaired the damage to the world, but at the cost of a vast amount of souls as well tempering of Convocation. These same tempered individuals then announce to sacrifice even more souls, specifically new life that they could use to replace the sacrificed souls. Not only is the sacrifice morally abhorrent, its a plan that could further empower Zodiark. And once again, its being suggested by a group of leaders who are knowingly tempered by Him. Thus a way to stop Zodiark is needed, one that would allow them to defeat a much more powerful being, while also not directly damaging or destroying it as its existence is now tied to the world itself. Thus, Sundering magic.