A fundamentalist Baptist preacher, who expelled a 14-year-old white girl from his Woodbridge private school for allegedly dating a black male classmate, testified yesterday that his religious beliefs taught him that God is against interracial dating and marriage.

Quoting portions of the Bible, Aleck Lee Bledsoe of the Marumsco Christian School told a packed courtroom in Alexandria that he was not aware that marriages between blacks and whites are lawful.

"I have never studied the law," Bledsoe testified. "I have studied the Bible."

Bledsoe testified in federal district court as lawyers for the young girl's father began presenting evidence that the minister and the school had violated his daughter's constitutional rights by expelling her.

Raymond Fiedler is seeking $70,000 in damages on behalf of his daughter, Melissa, in the case brought by the American Civil Liberties Union.

As the trial opened yesterday, lawyers characterized it as an unprecedented conflict between religious freedoms and civil rights. Emotions ran high on both sides and U.S. District Judge Oren R. Lewis had to threaten to clear the courtroom after laughter broke out among spectators during the minister's testimony.

Lewis said he would not allow "a mockery" to be made of the case. He said Bledsoe, a short, stocky man who wore thick glasses and a crew cut, and who testified with a toothpick between his teeth, has a right to his religious beliefs.

The judge is expected to rule on the case today after closing arguments.

Bledsoe said yesterday he first became aware that Melissa was associating with a black classmate, Rufus Bostic III, last December when the two attended a Christmas party together.

"We were under the impression it was a romantic relationship," Bledsoe testified. The minister said that although his school promoted racial harmony, it opposed interracial "dating."

But Melissa Fiedler, in long auburn braids and a navy blue jumper, told the court yesterday that the relationship was not romantic.She testified in a soft voice that Bledsoe instructed her not to "associate" with Bostic and recalled that the principal warned her that if America was taken over by another country, "interracial couples would be exterminated."

Raymond Fiedler, taking the stand, said he could not believe such "poppycock" was being taught in the school.

Victor M. Glasberg, Fiedler's attorney, tried to show that the opposition to interracial dating was based on Bledsoe's belief - not the teaching of the school or the doctrine of Bledsoe's Marumsco Baptist Church.

Fiedler testified that nowhere in the school's by-laws was there mention of interracial dating.

Under cross-examination by defense attorney I. J. Crickenberger, Fiedler told the court that his daughter's alleged relationship with Bostic may not have been to her "social advantage" but stated he would never discourage interracial dating.

Gail Bostic, a former teacher at the Prince William County school and sister of the black student involved, testified that Bledsoe's opposition to mixed marriages was not "a religious belief."

"I have heard the pastor express that he is against interracial marriage," Bostic told the court. "He told me the sex drive is greater in black men that in white men."

She said her brother withdrew from the school shortly after Bledsoe expelled both Melissa and her sister, Charlotte, from the school.

Mildred Bostic, the boy's mother, angrily told the court, "There's nothing religious" about Bledsoe's opposition to interracial dating. The only basis for the ban, she said, was to avoid the "social consequences."

But witnesses for Bledsoe testified that the preacher spoke out against interracial dating and marriage on several occasions, although no one could testify as to when and where.

Bledsoe, on the stand for nearly two hours, said the Bible contained several references to God's opposition to interracial dating.

Quoting the Book of Genesis, Bledsoe said the lines "God created firmaments and water" support his belief of separation between the races. He also recalled the story of Moses, who married an Ethiopian woman, which he said caused problems for "the children of Israel."

"We do not believe in interracial marriage based on that doctrine," Bledsoe testified.

On the school buses, he testified, the students were separated by sex. "The boys ride in front and the girls ride in back."

According to Ellis B. Hawkins Jr., assistant principal of the Marumsco Christian School who testified on Bledsoe's behalf, interracial dating and marriage "was forbidden by God."

But Rufus Bostic Jr., the father of the black student, testified that he understood the Marumsco Church doctrine to espouse "no segregation among Christian persons." Bostic, who was the first black member of the Marumsco Baptist Church in 1974, was made a deacon in May 1975. He testified that Bledsoe "never preached on interracial romantic relationships."

Bostic said he left the church in February, shortly after the Fiedlers filed suit against Bledsoe.

Testimony also revealed that the Marumsco Baptist Church, which once was attended by over 200 persons, has lost two dozen members and over 100 churchgoers since January.

According to Bledsoe's lawyer, the school filed for bankruptcy yesterday and probably will not open in September.

The Fiedlers said they have moved their home from Nanassas to Stafford County after a series of harassing phone calls and letters. "One morning we found a dead cat inside the mailbox," Raymond Fiedler, a $20,000-a-year technician for Washington Gas Light Co. said yesterday outside the courtroom.

Fiedler said he is not a social reformer. "I made waves.In this case, it's necessary."

Before adjourning yesterday, Lewis said from the bench. "It appears to me the facts are not disputed. The question is whether or not this school has the right to expel a student who doesn't comply with religious beliefs of the minister and head of the school."

That question is likely to go to the Supreme Court if necessary, according to Fiedler's attorney. CAPTION: Picture, JUDGE OREN R. LEWIS..."the facts are not disputed."