Stephen Rezza just dropped "Hard Mode" ... and we're told he got her family's okay to sample Christina for the track. Best of all for Christina's fans -- this is just the first of many new songs with her angelic voice.

Rezza's dropping a full album of his orchestral pop later this year, which is titled, "Songs for Christina." That's appropriate because every single track on it will feature either her vocals, her old music, or even voicemails from her.

It's a touching tribute to Christina. As we reported, back in June 2016, the singer -- who'd already appeared on "The Voice" -- was doing a meet and greet with fans in Orlando when a deranged man walked up and opened fire. He then shot and killed himself. Christina was only 22.

After her death, Stephen helped produced her posthumous album "All Is Vanity," which dropped in 2017. He then started working on "Songs for Christina" ... but that stopped when he suffered a seizure in 2019. Turns out he had an avocado-sized brain tumor. Surgeons successfully removed 95% of the mass.

Once he recovered, we're told Stephen had a new perspective and was determined to finish the album.

Stephen tells TMZ, "I almost never put these songs out. They're so personal I didn't think the world wanted to hear them. Next thing I knew I was on an operating table wide awake, skull open while my brain's being worked on. I almost lost everything. If I had died or lost the ability to make music, the only thing I would've truly regretted would've been never finishing this album for her."

BTW, Stephen's "Hard Mode" is now available on streaming services. It's about persevering when God flips the switch, and it's the only song on the album written after his operation.

Helluva way to persevere.

#RIPChristina