Sen. Jesse A. Helms (R-N.C.) is a man of firm convictions and no little parliamentary skill. So when he found out the man he and President Reagan want to become surgeon general was too old to hold the job under current law, Helms inserted an amendment into a credit card bill.

The credit card bill, amendment intact, sailed through the Senate early last month. It appeared Helms had scored a strategic victory and the decks were cleared for Dr. C. Everett Koop, an eminent Philadelphia surgeon and prominent anti-abortionist, to become surgeon general.

But yesterday the tactic boomeranged as a House subcommittee used the amendment to open a wholesale attack on Koop's qualifications as a public health officer, his understanding of the problems of women and homosexuals and his denunciation of pro-abortion statements by mainline Protestant denominations.

The pro-abortion statements, Koop wrote in a 1978 article in the Journal of the New Jersey Medical Society, illustrate "superficial theology, lack of morality and an insensitivity to the eventual reward for their depravity."

"I've never met Dr. Koop. But what I've heard about Dr. Koop, he scares me," said subcommittee Chairman Henry A. Waxman (D-Calif.). Waxman said he disagrees with many on the abortion issue, "but I don't say those who disagree with me are depraved."

Waxman, however, confessed some discomfort over holding the hearing. He said he is all for doing away with mandatory retirement ages, and normally would support doing away with the law that limits the age of a prospective surgeon general to under 64. Koop is 64 1/2.

But Waxman expressed outrage over Helms' amendment. It is "obviously an effort to install Dr. Koop as surgeon general," he said. "Why should a private bill for Dr. Koop he handled in such a surreptitious manner? What does the administration have to hide from us about Dr. Koop? Why does the administration hide Dr. Koop?"

It was a bizarre twist of parliamentary fate that the Helms amendment even made it to Waxman's health subcommittee. The credit card bill -- which would simply extend a three-year-olld law that bans surcharges on credit card purchases and allows merchants to give discounts to customers using cash -- had been approved by the House before it passed the Senate.

Normally, the House at this point in the legislative process would have had two choices: it could agree to the nongermane amendment or it could send the matter to a conference committee. But Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), upset with Helms' tactic, found an obscure House rule to refer the amendment to the health subcommittee. It was the first time the procedure had been used since 1951, according to House clerks.

Koop is deputy assistant secretary of the Department of Health and Human Services. The Reagan administration has indicated he would become assistant secretary and surgeon general under a department reorganization, a move supported by a host of anti-abortion groups.

No one questioned Koop's qualifications as a surgeon yesterday. Until recently, Koop was chief of surgery at Philadelphia's Children's Hospital, and he gained fame for a series of successful operations separating Siamese twins.

But a group of public health experts went after him on other grounds. A spokesman for the American Public Health Association, who said his organization had not opposed the appointment of a surgeon general in the last 100 years, said it opposed Koop because he has no training in public health or preventive medicine.

Dr. Margaret W. Bridwell, of the Women and Health Roundtable, said Koop's "public statements suggest that he opposes some forms of birth control such as the IUD [intrauterine device] and certain birth control pills, does not believe that unwanted pregnancies are a major public health problem, and ridicules those who have attempted to deal with the issue."

And Dr. Walter J. Lear of the National Gay Health Coalition said, "Koop appears to have strongly held beliefs about homosexuality which are not supported by established medical thought, practice or science."

Koop refused invitations to testify. A department spokesman at the hearing said he was in Philadelphia lecturing, and felt it unnecessary to testify because he has not yet been appointed surgeon general.

But he received support from several Republicans on the House subcommittee. Rep. Edward R. Madigan (R-Ill.) said he was "a little taken back" by the witnesses "who chose to assail the character of Dr. Koop rather than discussing the age question."

He then read off a lengthy list of the surgeon's honors, including seven honrary degrees and the French Legion of Merit, professional accomplishments, publications and volunteer activities in underdeveloped countries.

Waxman said the subcommittee would not act on Helms' amendment until Koop testifies.