"This time," Sister Renee Oliver (O.S.U.) wrote several weeks ago, "I want to get to you before you write your article on the president's proposal for tuition tax credits for parents with children in private schools."

She got to me the other day and, in a long and impassioned interview, laid out her case in support of the proposed tax credits.

Black leaders, said the associate director of Citizens for Educational Freedom, are "inadvertently hurting their own people by locking their children into a system that is destroying them. They should be espousing any way out--tuition tax credits, vouchers, whatever-- instead of blindly defending the public school system. Public school enrollments are down, the number of administrators is up, and the amount of money being spent is up, and yet they have a poorer product than ever before."

And how would she change that dismal picture? An important first step, she believes, would be enactment of the Reagan proposal, which would allow parents of non-public school students to deduct 50 percent of the school's tuition--up to a maximum of $500--from their federal income tax.

Even after our long conversation, I'm not sure precisely how she thinks the tax credit proposal would help those children who are getting the worst of public education: the children of low-income families for whom a 50 percent rebate at tax-filing time would hardly be enough to enable them to afford private school tuitions. The most obvious beneficiaries would be those parents whose children are already in private schools.

She concedes at least a part of the point. "True, tax credits will not help the poor unless a refundability clause is included, and even in that event they probably will not help the poorest of the poor. But it is not wise to hold back all progress until every single individual is included.

As for the fact that the most immediate help would go to those whose children are already in private or parochial schools, she sees that as simple constitutional justice. The present system, she says, makes "second-class citizens of parents who want their children steeped in moral values--second- class because they cannot share equally in educational tax money."

Her key point, though, is that our children's salvation lies in "the free, open market, the competitive free-enterprise system, without governmental interference," a system that tuition tax credits would open up to children now languishing in public schools. Given the tax credits, or, more ideally, a fullfledged system of educational vouchers, "I think (new) schools would arise to meet the needs of every child that is out there."

And why haven't those schools arisen already, particularly in Washington, where nearly every reputable private school has a waiting list comprising children whose parents have the money in hand?

"The economic climate is too tenuous," she says. "As long as they can get something for free, parents are not going to be as ready, when the financial crunch comes, to sacrifice for their children's education."

Would she require the private schools to accept children without regard to race or other arbitrary distinction? And if she would, wouldn't that increase, rather than diminish, the amount of "governmental interference"?

"I don't know," she admits. "The question is too big for me. I don't know what the government or anybody else should do about segregation. But I think that the de facto segregation that is going on in the present system far outweighs the danger of a few segregated schools."

What, in her view, is the crucial advantage of parochial schools?

"Moral education," she says. As evidence that public school youngsters are not being given adequate instruction in the moral values, she points to such things as school vandalism, thievery and physical violence--problems that tend to have a higher correlation with family income in the school neighborhood than with whether the school is private or public. Are different moral values being taught in the public schools in affluent neighborhoods than in low-income ones?

Perhaps, she says.

Then wouldn't open-enrollment public schools make a difference?

"Private school parents would still be second-class citizens," she says.

She returns to the question of race. "Historically, the black community has been very loyal to the public school establishment, hoping to achieve equality and a decent education for their children. But in reality, we have to admit that the public school system has failed the vast majority of black children."

If the public schools continue the academic gains that have been made in the past few years, would tuition tax credits become a less urgent concern?

"I'd still see the same urgency from the religious community's point of view," she concedes..