Answer:
First, the Bible never teaches an imminent return of Jesus, let alone that God uses it to sanctify believers. Imminent death, yes, but not an imminent rapture.
Second, Jesus in his Olivet Discourse teaches quite the opposite. He teaches that because his return is delayed with intervening events, he warned of the temptation for believers to become spiritually complacent. So his warnings to be vigilant because his return is not imminent is itself a sanctifying agent for the church.
Of course, the pretribber is going to object that Jesus is speaking of the rapture in the Olivet Discourse. They claim that Jesus is addressing so-called ‘tribulation Jewish saints’ referring to his second coming.
Pretribs also claim that placing intervening events before Jesus’s return “diminishes” vigilance. Here is where their argument falls apart (and I have never received a coherent answer from any pretribber): If intervening events diminish spiritual vigilance of the church, then why would it not also diminish it for tribulation Jewish saints!
