“Therefore, let not the one who believes not therein (i.e. in the Day of Resurrection) but follows his own lusts, divert you therefrom, lest you perish.”
Don’t you dare let anybody who doesn’t believe in the Hour get in your head and obstruct you. There are some people who, when we spend time with, make us forget about the Day of Judgment. The Ayah is not saying that they will make us disbelieve in the Hour. No, it’s not about disbelieving. We all believe in the Hour.
But when we hang out with the wrong kind of people, we stop thinking about it. And the more we hang around with them, we get so turned off from it that when someone tries to give you a reminder, you’d say “Can we talk about something less depressing?”
So why is this important? Because when someone stops thinking about the Hour, they stop thinking of the consequences of their actions. And when that happens, they will end up doing whatever they desire, whatever urge they have, even though that desire is completely filthy and impermissible.
This Ayah reminds us that if we allow such a thing to happen, it will bring us to our own downfall.