Was opium to him tylenol or fentanyl?

Yes. Those are both painkillers, and each has its use in medicine. If your gran was laid up in the hospital with a borken hip, they'd likely give her fentanyl for the pain. Funny enough, fentanyl is just a medical grade heroin (or, as it's also called, opium).

When Marx was writing, cocaine, alcohol, and opium were just about it for modern medicine. Of those, opium had the fewest side effects. It killed your pain and, as long as the dose wasn't extraordinarily high, it didn't kill you. That was it. Compared to alcohol or cocaine, it was by far the ideal treatment. Remember, penicillin wouldn't be discovered until the 20th century -- at this point in history, you'd be lucky if your barber-surgeon was cleaning his blades between patients, let alone washing his hands!

People knew opium got you fucked up, soothed pain, and could ruin your life if you let it keep seducing you. They learned pretty quick that laudanum junkies weren't generally successful people.

It can be both. Religiosity is a spectrum. You can be casually religious for the community and maybe adhere to some passages here and there (Tylenol - though it’s not an opioid) or you be someone who kneels down and sobs as he prays to the Holy Spirit and donates hundreds of dollars each Sunday (fentanyl)

It soothes your misery one way or another. You can still be aware of the material conditions and be a self proclaimed religious person.

Probably more like booze. A way to self medicate away the horrors of reality when there seems to be no other way to deal with them.