Being “lukewarm” is not being in a state of grace. It’s not sinning any notable and impressive mortal sins, and therefore thinking one is doing okay, because one is not seeing that nasty little mortal sins are also bad. You probably know people who think that as long as they’re not killing anyone, they couldn’t possibly be really sinning. It could also be a matter of sins of omission, when you know perfectly well that you should be helping someone who is close and in dire need, but you can’t be bothered to love or hate that person enough to do anything.
God wants us in our entirety. Sometimes we look for ways to quantify the amount of devotion and commitment we are required to give, so as to avoid trouble.
While it’s good to avoid things like mortal sin for sure, mere avoidance is not the fulfillment of our beatitude.
I think in lukewarmness we ask “how much am I required to do or give”, when the question is a non-starter because God asks us for just as much as he gives us. Everything.
I’m not sure that someone who was genuinely “lukewarm” would remain in the state of grace, as they would not care enough to do so.
You may have heard it said that great saints and great sinners are much alike. Both require effort and dedication. Lukewarmth is the opposite, someone who has neither the heat to seek Heavenly life nor the coldness to cringe from it, but only to “assume room temperature.”