When was the idea of all humans worth equal incorporated into Lutheran Christianity?

At least one scholar would say, this was the inherent message of Christianity, from the very beginning.

In Paul Among the People: The Apostle Reinterpreted and Reimagined in His Own Time, Sarah Ruden seeks to compare Paul's writings with that of the culture in which he lived. Over and over again, her project is to show how remarkably liberal Paul is when it comes to elevating the worth of those who were not considered fully human in Roman society. Whether it be the role of women (whom the culture would have seen as disposable sex objects) or slaves, Ruden advances the hypothesis that the modern notion of the fundamental worth of all individuals is the defining motivation of the Apostle Paul.

Her thesis is this:

Paul created the Western individual human being, unconditionally precious to God and therefore entitled to the consideration of other human beings. There is no sign that Paul intended all the social change that gradually (and sometimes traumatically) resulted, the development of the rights and freedoms that characterize the West. He laid down the law against only the worst abuses, such as pederasty, male promiscuity, and forced marriage. But broad social change did follow inevitably from the idea he spread: that God’s love was sublime and infinite, yet immediately knowable to everyone. No other intellect contributed as much to making us who we are.

Ruden, Sarah (2010-02-10). Paul Among the People: The Apostle Reinterpreted and Reimagined in His Own Time (Kindle Locations 180-181). The Doubleday Religious Publishing Group. Kindle Edition.

Slavery

In her analysis of slavery in the Greco-Roman world, for example, Ruden points out how radically different Paul's letter to Philemon is than society at large. Whereas the culture viewed slaves as pets and runaways as moral lowlifes, Paul radically reaches out to the slave-owning Philemon, urging him to view Onesimus as a brother. Ruden writes:

The letter to Philemon may be the most explicit demonstration of how, more than anyone else, Paul created the Western individual human being, unconditionally precious to God and therefore entitled to the consideration of other human beings

... as I wrote above, Paul had a much more ambitious plan than making Onesimus legally free. He wanted to make him into a human being, and he had a paradigm. As God chose and loved and guided the Israelites, he had now chosen and loved and could guide everyone. The grace of God could make what was subhuman into what was more than human. It was just a question of knowing it and letting it happen. The way Paul makes the point in his letter to Philemon is beyond ingenious. He equates Onesimus with a son and a brother. He turns what Greco-Roman society saw as the fundamental, insurmountable differences between a slave and his master into an immense joke.

Women

When it comes to the role of women, Paul is equally radical. Many feminists like to point out how "backwards" Paul is in demanding that women cover their heads, for example. Ruden points out this was actually something that enhanced the status of women in his day.

I think Paul’s rule aimed toward an outrageous equality. All Christian women were to cover their heads in church, without distinction of beauty, wealth, respectability—or of privilege so great as to allow toying with traditional appearances ... f the women complied—and later church tradition suggests they did—you could have looked at a congregation and not necessarily been able to tell who was an honored wife and mother and who had been forced, or maybe was still being forced, to service twenty or thirty men a day. This had never happened in any public gathering before. The new decree made independent women of uncertain status, or even slave women , honorary wives in this setting.

Or when it comes to divorce:

A marriage—or a divorce—for spiritual purposes was unheard of up until Christianity, and was entirely against Greco-Roman social norms. Paul’s vocabulary itself moves toward change. In verses 10 and 11 are the traditional terms, pointing to the spiritually poisonous traditional gap in power: the wife must not “separate” from her husband—or if she does, she must either stay unmarried or become reconciled to the same man—and the husband must not “divorce” his wife (literally “throw out,” the same word as for disowning a child). Under the laws of the Greeks and Romans, the wife could only remove herself, forfeiting her home and children; the husband, in a divorce, sent the wife back to her parents’ house with nothing but her dowry, and if she had neither parents nor a dowry, he could put her on the street. Nothing could entitle her to a share in his home or to a new one of her own

Sex

Ruden goes into great detail showing how coarse and abusive Roman society was. Divorce was a casual thing, Pedophilia and the rape of young boys was common place. Society encouraged virile men to objectify any beneath them. Ruden uses contemporary sources to show this, then remarks on how different Paul is. She writes:

For the polytheists, the essence of p**neia was treating another human being as a thing. If I had been one of Paul’s typical early readers, whatever else I understood from his use of the word, I would have picked up that treating another human being as a thing was no longer okay.

This theme is the theme of Ruden's work - that Paul's understanding of who Christ is is inherently the unintentional birth of the Western notion that all people are inherently eual in the sight of God.

C.S. Lewis, writing half a century before, also agrees that this notion of the eternal worth of the human being is inherent to the immortality promised by the Gospel. In Mere Christianity, he writes:

If individuals live only seventy years, then a state, or a nation, or a civilisation, which may last for a thousand years, is more important than an individual. But if Christianity is true, then the individual is not only more important but incomparably more important, for he is everlasting and the life of a state or a civilisation, compared with his, is only a moment.

Glibbly, one can also point to Galatians 3:28, in which Paul writes:

There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus.

She is under no illusions that Paul would have forseen the present situation - but she definitely would argue that our society has taken the radical Christian method of eequality to its natural conculsion.