PHILADELPHIA (AP) _ When Gloria Ahern arrived at her daughter's home about a week ago, she found an empty house, strangers at the door and family mementos in the trash.

The house, she learned, had been hurriedly sold, and her pregnant daughter, her son-in-law and grandchild had vanished.

A few days later, she said, her 35-year-old daughter called to say, '''Hi, Mommy. We're in South Carolina praising the Lord.'''

''I said, 'Linda, you're sick. Please come home.' And with that she hung up.''

Mrs. Ahern, of suburban Broomall, is among a group of 40 people in the Delaware Valley who believe their children have fallen prey to a Christian cult inspired by a southern radio evangelist called Brother R.G. Stair.

She said Stair's broadcast predictions of nuclear holocaust induced her children to sell their belongings for a song and seek shelter at his organization's farm in Walterboro, S.C.

Under his command or the dictates of his agents, his followers have abandoned their families and friends, signed over their funds and upended their lives, other parents said.

''That's ridiculous,'' said Stair, a 54-year-old self-proclaimed prophet, from his 74-acre farm on Thursday. ''There is no power that I have that compels people to do anything. I'm like a doctor or a lawyer. I can give my opinion, I can tell them what's best for them, and that's as far as I can go.''

''They have been under mind control, absolutely that's what it is, for months on end,'' Mrs. Ahern insisted. ''They were like puppets on a string. If they were told to do something they did it.''

Mary Del Guidice, also of Broomall, said she has put a lien on her daughter's house to try to stop her from leaving town.

''They think the world is going to end, we are all going to die and go to hell, except his people who go down south. There they are going to be safe,'' she said.

The parents have sought support from two area pastors and the Cult Awareness Network, a national non-profit group based in Chicago. The president of the Philadelphia afflilate, Marjorie Patton, said Stair's organization has all the characteristics of a cult, and she compared him to Jim Jones, whose followers committed mass suicide in Guyana.

''It's very important to recognize that the potential for tragedy is quite high,'' she said. ''When a leader has that much control over people, then you know they can gain more control.''

Stair, originally from Bethlehem, Pa., said the comparison was baseless. He said his 40 followers voluntarily chose a Christian life of self-denial in the country ''where the cost of living is cheaper.''

Law enforcement officials say they have found no illegalities in the cult's practices.

''Everybody that we've talked to who is associated with this religion is doing it of their own free will, and he (Stair) hasn't threatened anybody,'' said Greg Auld, FBI agent in suburban Newtown Square. ''They are freely giving over their possessions to the community.''

The three families who arrived from the Delaware Valley and the other followers are free to come and go, free to receive visitors and manage their funds in their own accounts, he said.

Together they believe that the economy will collapse in May; that President Reagan will not complete his term; and that nuclear war will envelop the world before the year is out, he said.

Chuck Rose, a self-described Christian counselor from Broomall now at the farm, said he has promoted Stair's teachings in the Delaware Valley for more than a year, although he first met Stair only within the last six months.

Rose said he announced his plan to join Stair, but exerted no pressure upon the nine families who comprised his congregation.

Mrs. Ahern's daughter, Linda McCourt, did not return telephone calls to the farm on Thursday.