PROBABLY THE best way to listen to "Messiah" is to lean back, relax and just enjoy its heavenly length, exulting particularly in your favorite passages-perhaps the elegant little "Pastoral Symphony" in the Christmas section, or such arias as "He shall feed his flock," "The trumpet shall sound" or "I know that my redeemer liveth." There are dozens of them, suitable for all tastes, in addition to the choruses that are the work's true glory.

But to better enjoy a performance, it is useful to know what "Messiah" is the most famous-if hardly typical-example of oratorio: a musical form derived from the marriage of religion and baroque opera that reached its highest development in the music of Handel.

The usual Handel oratorio is based on the lives of people like Saul or Solomon-blood-and-thunder stories about ancient heroes, almost impossible to distinguish from music that he wrote about such mythical characters as Hercules, such historical characters as Julius Caesar. But "Messiah" is unusual in its use of the New Testament and its significant theological content.

The work is divided into three parts, dealing respectively with Jesus' birth, his life and his resurrection, each blending solos and choruses and each rising to a final musical climax with the work's three great choruses: "His yoke is easy," "Hallelujah" and "Amen."

In performance, it should be an extraordinarily powerful work, from the opening overture to the concluding "Amen" chorus, which is probably the most spectacular and certainly the best-loved setting of a single four-letter word in all of music. Handel was essentially a dramatic composer, and even though there is only occasional narrative content in "Messiah," he makes the most of each opportunity. Sometimes his musical elaborations amount to little more than a play on words. In "Thus Saith the Lord," for example, he introduces a quivering vocal ornament very descriptively called a "shake" on the word "shake."

But elsewhere the musical effect reinforces marvelously the feeling and the picture implicit in the words. The stumbling bass line of "The people that walked in darkness," for example, gives a striking effect of walking in darkness; and the vocal lines go splendidly astray in "All we, like sheep, have gone astray."

Even more striking than the pictorial effects are the emotional ones, from the pure delight of "For unto us a child is born," which should be performed with an angelic lightness of texture, to the fury of "Why do the nations, which is a "rage aria," one of the standard features of baroque opera. Handel is often at his best when he is least elaborate-in "He was despised," for example, which is one of the few solo arias that he left untouched throughout his life.

In a modern performance, there are a number of aspects to watch for. You are free, for example, to sniff at a conductor who introduces crescendoes into the music. This device was developed at the court of Mannheim some years after "Messiah" was composed, and created a sensation in Europe too late for Handel to exploit it. Look with suspicion also on ritards, the slowing down of tempo at climactic points in choruses. In a few places, Handel has written his climaxes in longer notes than the preceding passages, but usually (notably at "wonderful counsellor" in "For unto us a child is born") he expected the music to continue at the established pace without this sort of meddling.

Handel shows his theatrical sense in the way he builds climaxes, particularly in the three great choruses. Of these, the central one, "Hallelujah," is certainly the best-known and encrusted in folklore-and also the most often abused, even by conductors who have made an effort to perform the rest of the music in a proper baroque style.

In the first four syllables of this chorus hit the audience like a sledgehammer, it is being done wrong and the chorus will probably not be able to make the proper effect a few minutes later when the music reaches its true climax with all the voices united in singing "King of kings and lord of lords." Handel's instructions indicate that it should begin at a moderate volume-with only part of the orchestra playing the introductory measures.

Most performances today try to sound as much as possible like those of Handel's time. Generally, such efforts are doomed to partial success at best: It is easy enough to get approximately the right number of performers, but a lot harder to get the correct style.

An all-out purist performance (unlikely these days) would include about half a dozen boy sopranos in the chorus and probably at least one adult male (surgically altered or specially trained) singing solo material now usually assigned to women. Handel used both women and castrati as soloists according to availability and quality of performance.

In addition, all the stringed instruments would be equipped exclusively with gut strings and would be tuned almost a half-tone lower than usual; standard pitch has crept slowly upward during the last two centuries, and is still doing so. The conductor would not stand on a podium with a baton but would direct from the harpsichord during solos and the organ during choruses.

But the greatest challenge in recreating what Handel might have heard is the incorrigible tendency of modern musicians to play or sing exactly what they see written down. Handel and his contemporaries would not have understood this at all. Performance in his time was a mixture of conventions and liberties that were lost a generation later when music changed from baroque to classical. These conventions have been much-studied in the last generation, and the audience at a "Messiah" that aspires to authenticity has a right to expect them. The most important are undoubtedly the double-doting of the overture-a special, irregular rhythm developed by the French in which the long notes are held longer, the short notes shorter than the way they are written.

Also being revived is vocal ornamentation. Sometimes, even in Victorian reworkings of the score; there are "ad lib" markings in a vocal part, where the singer is invited to add an embellishment-an invitation which was automatically refused for many generations. But even when the invitation is not written down, this kind of ornamentation was regularly added - on long-hled notes, for example, and at the end of cadences. And in da capo arias such as "He was despised," where the first section is repeated at the end, it was taken for granted that this section would be sung rather simply the first time and that ornaments would be improvised during the repetition.

The sound of the orchestra, finally, would be a little bit more windy than that of a modern symphony orchestra. Handel usualy didn't bother to write out the parts for oboes and bassoons, which simply doubled the parts played by strings; but he did keep a record of payments made to players, and from these records it is clear that his orchestra had a higher proportion of woodwinds than we expect today. A lot of them were probably a bit out of tune, too, but authenticity can be carried too far.

There are numerous fine recordings of "Messiah," as these should be for a composition of such stature and popularity, and several are at a level of quality that makes a choice among them largely a matter of personal taste. Among the international all-star performances, my favorite for fervor, musicianship and idiomatic baroque style is the one conducted by Charles Mackerras (Angel SC 3705, three records), but I cannot quarrel with anyone who chooses the rather breath-lessly-paced version of Colin Davis (Philips 6703 001, three records), or the Leppard version (RCA CRL 3-1426, three records), which takes the musicological middle of the road.

For those who prefer tape, there are two excellent editions. On Advent cassettes (EE 1061, two cassettes), there is a performance by the Handel and Haydn Society of Boston, with soloists who are good but not quite as stellar as the big international casts on the major labels. A single cassette of highlights has just been issued, and it deserves special attention because it uses intelligently the chief resource of the cassette-potentially longer playing time-with approximately 45 minutes of music on each side.

But the "Messiah" I have been playing most often and with deepest enjoyment this season is the new open-reel tape version from Barclay-Crocker (11 Broadway, New York 10004). It contains the excellent performance recorded for Argo Records under the baton of Neville Marriner, but the performance, fine as it is, takes second place to the attractions of the sound-pure, natural, totally free from distortion and startlingly realistic.