IF YOU THINK about the MX -- and how could you not? -- think about it as a Christmas missile.

It isn't just its seasonal name -- "Peacekeeper" -- or even that it's what the president wants most from Santa.

Start with its plight. We all know that the basing mode was a terrific problem with the Nativity. Joseph and Mary were turned away from dense-packed hotels in Bethlehem, and had to improvise. The Pentagon planners could be humming "Away in a Manger" as, back at the drawing board, they try to find a crib for MX's bed.

When you are talking homelessness -- and many people who live in their cars were doing that on Capitol Hill this week -- you are talking Peacekeeper. It's been evicted from 34 proposed dwellings already.

I wonder if the Joint Chiefs have applied to the Salvation Army. It has an excellent shelter program.

I first knew that MX had Noel written all over it the day I listened to the president's science adviser, Dr. George A. Keyworth, at the Senate Armed Services Committee. He was talking, I think, about the timers Soviet missiles have. He mentioned "terminal homing."

Isn't that a lovely phrase? Isn't it just another way of saying, "I'll be home for Christmas?"

I don't know exactly what "terminal homing" means on a Soviet ICBM as it speeds toward its rendezvous with our warheads in Wyoming. But "terminal homing" is what is going on all around us now -- people jamming terminals, eagerly undertaking fatiguing travel just to be present in the family circle around the fireside.

Another beautiful, pastoral phrase comes up in Peacekeeper terminology. George Wilson, who has mastered its special vocabulary, wrote the other day about "low-grazing Soviet missiles."

Makes them sound like sheep, doesn't it? Puts you in mind of the manger, the shepherds -- "the cattle are lowing" -- the whole scene.

The nice thing is that the sheep missiles may skip off the target like flat stones skittering across a pond. That means that our MX, tucked deep in its coffin, would be secure. The low-grazers wouldn't hit Wyoming, although they might blow up California when they stop skittering. But Christmas is a time to forget workaday details of that nature.

The Air Force talks about a "pathfinder missile" which makes you want to go right out and deck the halls.

The context, in Pentagon testimony, may be a bit grisly, but don't go away.

"The detonation (of the Soviet missile) raises ejecta, which consists of large boulders, some of them larger than a Volkswagen, that are thrown up and ejected out of the crater for some significant distances."

The Pathfinder acts as an escort for the Peacekeeper, as it sets out on its retaliatory strike at Moscow. It will go first "to determine if a safe dust and debris environment existed after the MX field has been attacked."

The Pathfinder is a pretty parallel to the Star of Bethlehem, which guided the wise men. The wise men, the first Christmas shoppers, are admirable in many ways. On hearing of the birth of the Prince of Peace, they saddled up and set forth. Some of us wish they had mislaid the gifts, as us addled types do so often at this time of year, but that is another matter.

I hope the president is not too chagrined that the MX will not, after all, be under his tree. Christmas, for all its joys, can be a time of disappointments. He should think of the woman, dreaming of a diamond drop, who is gifted with a selection of imported cheese; of the man who wants a video game who finds a snow shovel in his stocking.

Yes, it is a time of wassailing, homing and giggling under the mistletoe. But it is also a time when the "window of vulnerability" in human affairs is wide open. Christmas cards bring greetings, but how often, too, tidings of death, divorce or the defection of a child to a cult requiring temple-charring in Sri-Lanka. Such news inevitably arrives just as you have mailed off your certitude of the recipient's well deserved earthly joys. Composing the message of condolence cuts seriously into resting merry, gentlemen.

Come to think of it, maybe the Pentagon planners should abandon their search for a new basing mode and go to work on some kind of Christmas card synchronization scheme: people in the first half of the alphabet to mail on a certain date, while M to Z holds off until later. That way, 50 percent of the population could reply rationally to its cards. Next year, the order could be reversed. Okay, it's not perfect, but haven't we been saying what is?

Betimes, I wish you an M X. That's short for "Merry Xmas," which is, the president tells us, all that Peacekeeper was trying to say.