THERE IS A FINE irony in the fact that Menahem Begin's theological bent, previously regarded as an obstacle to a Mideast settlement, may turn out to be an asset. The Israeli prime minister remains determined to treat the historic land of Palestine as a sacred entity. But instead of claiming that Israel must occupy or eventually annex the parts of that land captured by Israeli forces in 1967, Mr. Begin now suggests that, as long as Israel's security interests are cared for, it is enough to maintain in those parts the right of settlement by Israelis or, in his phrase, "Palestinian Jews." He would grant a similar right of settlement within Israel proper to "Palestinian Arabs" - this would seem to address long-standing Palestinian demands for the right of "repatriation." To this "mutual right of settlement" Mr. Begin gives the term "symmetric justice," as he put it on "Face the Nation" yesterday. This is a nice way of describing the test that any final settlement must be able to meet.

Mr. Begin is to present this idea, and others, to Anwar Sadat personally in just a few days. He offers his views on Palestinian Arabs as proof that he is not trying to lure the Egyptian president into a separate peace. At this point, no higher proof can be demanded of him. It must be kept in mind, however, that Israel cannot negotiate with Egypt the kind of West Bank settlement that Mr. Begin outlined on television yesterday - or any other kind. Until a suitable Palestinian entity actually comes forward to replace the PLO, which has now been repudiated by Israel, Egypt and the United States, Mr. Begin's offer of "self-rule" or "Autonomy" to Palestinian Arabs is an intriguing but untested possibility. Another prerequisite is that Jordan find the political room to move from the wings to the center of the negotiating stage.

Mr. Begin will probably continue to characterize his latest thinking as "traditional." So be it. If his new formula draws the Arabs into negotiations, it will hardly matter whether it represents a departure from what his previous views were thought to be. Mr. Begin would be the first to say taht the Sadat initiative and the Israeli response to it have made possible formulas that were unthinkable before.

President Carter, after meeting twice with Mr. Begin, hailed his "constructive approach." But as Mr. Begin himself said on TV yesterday, he did not come seeking Washington's "approval" - which would imply that Israel is a petitioner - but rather its "good will," which, mr. Begin said, he did in fact receive. In heading for a second meeting with Mr. Sadat, he is moving at a pace and in an atmosphere suggesting that Egyptian-Israeli negotiation is becoming positively intense.

The Carter administration, in its public statements, now seems to be doing everything that Israel and Egypt could propesrly ask. True, to judge by certain high-level press leaks over the weekend, some officials think that Mr. Begin in particular is sometimes inclined to "use" the United States for his own purposes. That's fair enough; Mr. Sadat's exploitation of his access to the American media suggests that he, too, is not indifferent to the importance of the influence that the United States can bring to bear on the bargaining process. But as long as both men appear to be moving toward an agreement, then there is no place for American complaint and no reason for American hesitation on the question of guaranteeing the ultimate settlement terms - a possibility introduced by Prime Minister Begin yesterday. It goes without saying that the conflict in the Mideast cannot be settled on any terms other than those that the parties directly involved agree upon.