Read the Letter D4vd Wrote to Fellow Inmate While Behind Bars

D4vd apparently made a pen pal while behind bars ... one whom it appears he initially agreed to help bail out -- before backing out of his promise.

We've obtained letters D4vd sent to a "trustee," an incarcerated person who works inside the prison, who worked at a jail in Los Angeles. This trustee met D4vd while serving food in the joint ... becoming friendly before D4vd started writing him letters. We're told they would mail out the letters even though they were in the same facility.

You can read the letters for yourself ... they are religious in tone with D4vd writing that he's putting his faith in God and telling a fellow inmate to do the same.

He includes specific Bible quotes like Isaiah 41:10, which states, "So do not fear, for I am with you; do not be dismayed, for I am your God. I will strengthen you and help you; I will uphold you with my righteous right hand."

D4vd says his promises of prayer aren't empty either ... assuring the fellow inmate he's actually praying for him and his family during their difficult time.

In another letter, D4vd appears to say he spoke to his lawyers ... and they don't want him giving over any money to this trustee, even though he'd previously promised he would.

We've learned the trustee needed $1K to make bail so he could get out of jail for a surgery his brother needed ... and D4vd allegedly promised the money to him. However, it seems he didn't actually give him any.

Our sources confirm these two had a friendly relationship ... with the pair even singing a jingle back and forth as part of an inside joke.

As you know ... D4vd is being held behind bars without bail as he awaits his trial. During a series of hearings last month, prosecutors introduced shocking evidence against D4vd -- including graphic sexual photos of his alleged victim, Celeste Rivas.