About midway through President Reagan's engaging New Year's Day address to the Russian people came a brief passage that must have been a stunner for the average TV viewer in Gorky or Vladivostok. The president described a society -- ours -- where people are free to think, speak and write as they like, free to send private or mass messages without intrusion from government officials.

"Our democratic system," he said, "is founded on the belief in . . . the rights of the individual -- rights such as the freedom of speech."

For Americans watching the Reagan speech, this was hardly a stunning assertion. Of course we have freedom of speech; of course we have the inalienable right to read and write what we choose, free of censorship or imprisonment. Those battles were all settled 200 years ago, weren't they?

In fact, the battles over these basic rights are not all settled. And anyone who uses the personal computer as a communications tool -- that is, anyone who has ever called Compuserve, The Source, or any of the thousands of other computer "bulletin boards" now thriving in our country -- is at the vanguard of the most important free-speech struggle of our age.

So here is a political issue that all computer users should watch closely in 1986: the challenge of keeping electronic speech free, outside the grasp of government types who want to regulate and control computer communications.

The emergence of the computer bulletin board is one of the revolutionary developments in the history of free speech -- a milestone on a par with the work of Gutenberg, Bell and De Forest. A computer network is the cheapest and most accessible mass communication tool ever devised. You may not be able to afford a set of presses like those that turn out each day's Washington Post, or broadcast facilities like those at CBS News. But if you have a telephone and $150 worth of computer gear you, too, can spread your message to tens of millions of people.

If Patrick Henry and Tom Paine were around today, they would be distributing their pamphlets via computer networks. As Prof. Ithiel De Sola Pool notes in "Technologies of Freedom" (Belknap Press, $8.95), "Computers . . . . are technologies of freedom, as much as was the printing press."

Some of our government "leaders" (including many so-called conservatives, who purport to dislike big government) blanche at this enormous new distribution of mass communication power -- power that remains largely free of government control.

And so we see a host of bureaucratic power grabs along the lines of S. 1305, the legislation proposed by Sen. Paul "Big Brother" Trible, a Virginia Republican who calls himself a "conservative." This bill would authorize federal bureaucrats to read (electronically) over your shoulder whenever you call a bulletin board.

Trible never has been accused of being a mental giant, but he thinks he's smart enough to decide what you should be reading in the privacy of your own computer room. If you call a bulletin board that carries what Trible considers to be "offensive material" (the senator's phrase) you'd be a criminal under the provisions of S. 1305.

The same impulse prompts local prosecutors to seize the computers and rifle the files of bulletin-board operators and teen-age "hackers." These seizures often stem from sheer ignorance -- like that of the Middlesex County, N.J., prosecutor who seized some teen-agers' Apple IIs on grounds that the kids were sending messages that could "move satellites about in space." Officials later conceded that this charge was groundless. But the precedent has been set: If the government doesn't like your message, it will seize the tools you use to send it.

Nobody would stand for it if a U.S. senator proposed that the government approve all the books you read in your own home or if a local prosecutor seized a daily newspaper's presses. Neither can we stand by quietly when the same Big Brotherism is brought to bear against electronic forms of speech.

The First Amendment that President Reagan boasted of on New Year's Day must apply to all speech -- written, spoken or electronic -- or it will eventually apply to none.

"The handling of the electronic media is the salient free-press problem for this decade," Ithiel de Sola Pool wrote. "As computers become the printing presses of the 21st century . . . speech will not be free if these are not also free.

"The onus is on us to determine whether free societies in the 21st century will conduct electronic communication under the conditions of freedom established for the domain of print through centuries of struggle, or whether that great achievement will become lost."