Literally after they escape with Jakku with BB-8 on The Falcon there is a scene in Snoke's chamber where he is pissed that the droid is going to get to The Resistance because they'll find Luke and Luke will come back and re-establish the Jedi order, up until that point they'd been operating quietly, now and Snoke literally says this:"Our strategy must change" that;s when Hux says the weapon is ready and they decide to blow up the Senate, that was their plan B, because now everyone knows they exist, they committed an act of war that exposed them for what they are in order to try and stop the Resistance form getting to Luke, they are that terrified of him. They even say that they are wiping out the Senate to take away Leia's allies and make it easier for The First Order to stop The Resistance from getting to Luke.
And then after Ren fails to acquire BB-8 because he thinks Rey is enough and she escapes they go to yet another chamber conversation where Hux reems out Ren for not getting BB-8 as well and tells SNoke that the droid is probably in the hands of the Resistance, And thus Snoke declares and again I quote "The Resistance must be destroyed before they get to Skywalker". And that's when they decide to charge up and target Leia's base which they found by following her recon back to her locatio.
That's two blatant expository sit and talk scenes that spell out exactly that the quest to find Luke is the centre of everything.
Why they tell Ren to not let his personal feelings interfere is that Snoke and The First Order would settle for destroying BB-8 and thus stopping Leia from finding Luke even if it meant never finding him themselves, something Ren doesn't want, he wants to find Luke.
The ends result is Snoke and The First Order are terrified that they will find Luke bring him home and he'll re-start the Jedi order. They were so terrified that they blew up the fucking Senate to send a message and thus exposing themselves and then put their entire weapon at risk by targeting Leia directly,
It was all to stop them from finding Luke. They literally say it twice, like not even implied but flat out said twice in plain english during giant exposition scenes, they attack the Republic so as to weaken the Resistance so that they can stop them before they reach Skywalker (literally paraphrasing Hux here) and then they try to launch on The Resistance base because if they don't wipe them out fully right away they will get to Skywalker. Everything they do is about stopping them from getting to Luke, because they don't want the new Jedi to rise.