As a result of recent judicial decisions, the hole in the church-state dike is widening. But if the United States Supreme Court decides next term in favor of the State of Michigan in the Grand Rapids ''shared time'' case, the constitutional church-state wall of separation is liable to be swept out into the current. Frankly, it's vital that this does not happen.
A key test will come in the fall when the high court wrestles with School District of Grand Rapids v. Ball to determine the legality of a Michigan program of public aid to parochial schools. Under the Grand Rapids plan - which began in 1976 and eventually spread to 18 other school districts in the state - 470 public school teachers have been sent into 41 private schools (the large majority of them Roman Catholic) to teach remedial and enrichment courses. Parochial classrooms have been temporarily and magically transformed into public teaching areas by removing religious articles from them and designating their 11 ,000 pupils ''part-time public school students.'' Taxpayer costs: $3 million-plus.
Parochiaid - or government funding of religious school programs - is fast becoming the priority concern of civil libertarians and other First Amendment protectors determined to prevent church-related intrusion into the public arena. Many see this issue as even more important to the integrity of public schools than the debate over school prayer, which has captured the church-state focus of late.
In addition to the Michigan case, the Supreme Court will almost certainly rule on the constitutionality of ''moment-of-silence'' school observances soon, perhaps next year. But the impact of cases where public funds are used by school districts to teach private, usually parochial, school students is likely to be more far-reaching.
The Michigan program was challenged on constitutional grounds by a group of Grand Rapids taxpayers. And two lower courts upheld the argument that the arrangement constituted unwarranted entanglement of religion and the schools. Chief Judge George Edwards of the US Sixth Circuit Court of Appeals minces no words in issuing a warning: ''A Supreme Court decision in favor of the Grand Rapids shared-time program would effectively destroy church-state separation in education.''
Hyperbole? A coalition of school and liberties groups - the American Civil Liberties Union, Americans United for Separation of Church and State, the National Education Association, and the National Committee on Public Education and Religious Liberty, among them - doesn't think so. Albert Dilley, attorney for those opposing shared-time, says high-court approval of the Michigan plan could prod religious groups across the US to set up ''day schools,'' with government picking up the tab for ''secular'' courses. Mr. Dilley even sees these newly designated ''public schools'' resulting in a government-sponsored dual system of education, ''segregated by religious denomination and race.''
The Reagan administration, which has already filed a friend-of-the-court brief on behalf of the Grand Rapids plan, has no such qualms. It argues that shared-time has only ''secular'' purposes (educational enrichment) and is totally under control of the public schools. The US Catholic Conference, the Catholic League for Religious and Civil Rights, and a group representing Orthodox Jews agree.
Shared-time advocates hope the high court will see the program as a proper ''accommodation'' of religion - rather than a constitutionally forbidden ''establishment'' or promotion of it. In a precedent-setting case this term, the justices allowed the erection of a religious Christmas display on municipal grounds in Pawtucket, R.I., on the ''accommodation'' basis.
Another chip in the dike came a year ago when the Supreme Court refused to outlaw a Minnesota program granting tax deductions to parents to offset private (including parochial) school tuition costs. The rationale was that the allowance was theoretically also available to parents of public school children. (Of course, there is no tuition for tax-supported education!)
Some court-watchers say the Grand Rapids case may be heard in the high court in tandem with a case involving a New York plan - recently struck down as unconstitutional by a lower federal tribunal - which provides public instruction to educationally disadvantaged children in private schools. This program is also staunchly defended by the Reagan administration as scholastically essential and unencumbered by church-state entanglements. But Circuit Judge Henry Friendly, in disallowing it, saw the risk ''... that school board personnel will act, even unwittingly, to foster religion.''
In 1975, the US Supreme Court set a standard in ruling against providing public personnel to church schools. That decision should stand. Parochiaid, for whatever reason, is not a good idea. It puts an unfair burden on the public sector, which is required to maintain religious ''neutrality,'' and unnecessarily invites meddling of the state in religion.
The First Amendment protects precious freedoms. This kind of assault on it must stop.