Yesterday was a big day in the history of the National City Christian Church. Not quite as big as the Sunday morning that President Lyndon Johnson first walked in the door, perhaps, but almost.

Yesterday the church dedicated the towering, 25-foot-high stained-glass Lyndon Baines Johnson Presidential Window, which features, among many of its symbols, an angel holding a space rocket.

And at a time when some people fear that the legacy of Johnson's Great Society programs will be swept away in the prevailing political winds, the church on Thomas Circle was filled with more than 1,000 of the faithful, including the Johnson family, several congressmen, former Cabinet members and hundreds of the parishioners to whom Lyndon Johnson became a familiar sight during his five years in the White House.

"I'm an Episcopalian," said Lady Bird Johnson before the ceremonies. "So at first, I tried to take Lyndon to my church. But then we met Dr. George Davis, and we began to go to the National City Christian Church more and more. It was a warm harbor in troubled times."

Former secretary of housing and urban development Patricia Roberts Harris read the Old Testament verses from Isaiah which included a favorite line of LBJ's: "Come, let us reason together."

"This window," said retired minister Davis from the altar moments later, "looks out on the dark corridors of human needs and cries of anguish. It is dedicated to a man who really did make a difference."

The dedication service took little more than an hour. Those in attendance watched Lady Bird Johnson accept a watercolor painting of the window from senior minister William Howland, and everyone listened to hymns sung by the East Texas State University Chorale. Afterwards, the crowd trooped downstairs for a reception of home-baked cookies, punch and piping hot coffee. Among those in the receiving line waiting to shake hands with Mrs. Johnson and George Davis were lawyer Abe Fortas, former postmaster general Marvin Watson, former national security adviser Walt Rostow, former mayor Walter Washington and Rep. Jake Pickle (D-Tex.).

Virginia Lt. Gov. Charles Robb circulated with his wife, Lynda. They shook hands heartily with one and all, in preparation, no doubt, for next Thursday, when Robb will announce his intention to run for governor. Luci Johnson, visiting from her home in Austin, was busy exchanging news with friends of the family.

Earlier in the afternoon much of the Johnson clan at prime rib and traded Texas talk with about 450 members of the Texas State Society. House majority leader Jim Wright (D-Tex.) was among those who came forward to greet the Johnsons. On the way out, not overlooking St. Patrick's Day, he added, "The Texans and the Irish have a lot in common, you know. We share the same-sense of innocent chauvinism, the same drive to be among people of our own kind. In a setting like this we can be cousins together. And anything with the Johnsons is like a family affair."

The family feeling carried over to the church reception, and so did the concern for social welfare programs. "What I believe in is budget figures," said Harris. "And I say, instead of pushing people into hysterical decisions, 'let us reason together.' Maybe there's hope, though. I saw so many of the old battlers out in the church aisles today."

"I don't think the turning away from the Johnson programs is permanent," said Fortas confidently. "How could it be? Those programs are the future of the country, the future of mankind."

Over in the corner, Pickle, who holds the congressional seat LBJ once did, watched the proceedings with aplomb.

"Remember the cartoon of the president walking toward two budget fellas? One of them sees him approaching and says, 'When LBJ says, "Come, let's reason together," watch out!'"

The fact that Lyndon Johnson professed no great love for solemn spectacle during his lifetime was mentioned more than once during the afternoon, which gave rise to speculation as to what he would have thought of the afternoon's proceedings. Pickle had an idea.

"Why he'd probably take one look at the goings on, walk right up to one of those college prosidents over there, and try to get an education bill going."