Standing beneath the ornate gold and marble altar of St. Patrick's Church, Jerome Scardina, 51, reverently placed the communion wafer on the tongue of the worshpier standing before him.

Serene and composed, only a slight slurring of his words as he spoke the ritual words gave any clue that the man, who moments earlier had been formally recognized as an extraordinary minister of the eucharist in the Roman Catholic Church is a victim of Down's syndrome and has been mentally retarded from birth.

Scardina's installation yesterday as an extraordinary minister marks the first time the church in this country handicapped person for that role, which the church created eight years ago. Extraordinary ministers are lay persons specially trained and entrusted with the most scared elements of Catholic worship rites. They assist the priest in holy communion and mass.

The smile on Scardina's face at a reception after yesterday's rite - and the happy tears of his sister, Delores Rebuck - reflected the revolutionary changes in the attitudes of the church toward the mentally handicapped in recent years.

Because of his affliction, which is more commonly known as Mongolism, he was not permitted to make his first communion until he was 27, instead of the usual age of 7 or 8.

Scardina's sister recalled the anguish and frustration of that earlier time. "The priest told my mother that if he could learn to say the Our Father, the Hail Mary and the Act of Contrition he could make his first communion," explained the sister. "I can remember when I was 8 years old, down in the basement, trying to teach him his prayers. But he can't memorize things."

The Baltimore archiosese has an unusually extensive program for mentally handicapped persons. Scattered throughout the diocese are 25 special education centers for children's mass at which auxilliary bishop J. Francis Stafford formally recognized Scardina as extraordinary bishop, he also confirmed seven mentally handicapped adults. the oldest of whom was 65.

In a brief homily, the bishop addressed the confirmants.

"God is not somebody who is going to hurt you but he is like a father," he explained to them. "Always when you pray to God, pray to him as a father.

Bishop Stafford told the confirmants to pray twice a day, in the morning and in the evening. "When you get up, just say, 'Good morningz, Father Heaven,'" he said. "And in the evening just thank Him for the day."

The Bishop told the congregation of about 125 relatives and friends of Scardina and the confirmants that "What is going on here today is the essence of what Christianity is all about...it shows the love of the least of God's people."

The confirmants received their first communion from Scardina. "Now they have one of their own to serve them," explained Sister M. Justa Walton, director of the diocesanwide program for the mentally handicapped.

Scardina is expected to serve mainly for the handicapped, but under the rules of the church he is entitled to serve at any mass where lay ministers may be needed.

One of Scardina's staunchest supporters is a childhood friend and fellow member of St. Patrick's parish, Peggy Cushing, who herself has a daughter with a handicap.

"We felt the church was discriminating against the handicapped." she said. "You don't see retarded people as ushers, for instance. I believe they have this right."

Scardina's plight came to her attention three years ago when his mother died. "His best friend in the world was gone," she recalled. A parish prayer group of which Cushing was a member "reached out and became his family," she said.

In the prayer group Scardina was noted for his spiritual depth, she said. "When Jerome prays, his face lights up. He is very holy man."

The selection of Scardina as an extraordinary minister is in part an attempt by the church to give more visibility to the mentally handicapped. "The parish has accepted him," Cushing said. "He's like a beacon of hope for the others...after all, there are no rejects as far as the gospel is concerned. It's about time we started acting that way."