The idea of enslaving foreigners and indenturing people of your nation - that is to say those who share a common culture, ethnicity, religion, etc. with you - is old. Leviticus 25;

^44 Your male and female slaves are to come from the nations around you; from them you may buy slaves. ^45  You may also buy some of the temporary residents living among you and members of their clans born in your country, and they will become your property. ^46  You can bequeath them to your children as inherited property and can make them slaves for life, but you must not rule over your fellow Israelites ruthlessly.

I certainly don't mean to make this religious (or blame the Old Testament for the barbarity of the Atlantic Slave Trade) but instead offer it as confirmation of the idea at a prior point in time. Leviticus is the third book of the Bible and was written before 300 B.C., so the idea has been around much longer than European nations of relevance here. When did race based chattel slavery of Africans by those nations begin is another question entirely, and I think that's what you're asking.

In the mid 1400s the Pope issued a Papal Bull resulting from a "trade" voyage taken by a Portuguise Captain in which he returned with a human cargo of captured Africans. Slavery by Europeans of Africans in what became the Atlantic Slave Trade had begun (though it was still debate and opposed by some), but it was based on their "heathen" status and not their race/ethnicity. This was equally applied to Native people from the Azores and Ireland to the Caribbean and Americas. This kept on for a long time and with all colonial powers but in the mid 17th century in English colonies this all began to change, and this change came for a few reasons.

Legally there were challenges to both the statue and common law that actually won in court. Perhaps the most famous is that of Elizabeth Key Grinstead, the daughter of a white ship captain and an enslaved African woman. She was baptized by her father who, under English law, had to provide for her. He placed her in an indenture but she was held past her terms, then claimed as a "life indenture" by the holders. She had married another indenture, a white male, who was released upon his term ending. He sued on her behalf and won, but died shortly after. Both Elizabeth and her son had been freed by the efforts of the father and by the laws of England - something had to change.

Partus sequitur ventrem is a legal doctrine that, up until the mid 17th century, had been applied through Roman law to livestock. It means "birth follows the womb" and it was soon utilized to bypass English law, which left the father on the hook for claiming the child and providing for it, in order to render the children of enslaved women and white men indistinguishable from those of enslaved men and women, legally speaking. This was 1662 in Virginia. From there a series of legislative acts in the Mid Atlantic Colonies established a defined race based chattel slavery system, and this was solidified by legislation in the first decade of the 18th century.

Further south, the Caribbean Colonies began to suffer land shortages, particularly when Freedom Dues became, well, due, which occurred at the conclusion of the indenture contract. Some tools, some seeds, some clothing perhaps, and a chunk of land. But the indentured stopped dying and started collecting, which led to South Carolina being formed (1670, right in the middle of this) and giving away huge headrights - massive tracts of land to the freeholder, and more land was included for every enslaved soul brought or later imported. Then they jumped on board with the idea that religion and baptism don't matter, nor does the paternity while still in a patriarchal legal system that never allowed for enslavement. Irish and Scot were sent here but could challenge in court to petition against unreasonable treatment - enslaved (Africans and Natives) could not sue - and this is why there was not "Irish savery" as they were legally humans. Now what Elizabeth's troopers did to them is another tale, but somewhat relevant is the mention that she granted Humphrey Gilbert rights to conquer lands not held by a Christian Prince north of Spanish Florida after he lined his pathway with the skulls of Irish resisters conquering them, and the man that coined the name Virginia in honor of Elizabeth - the Virgin Queen, Walter Raleigh (Gilbert's half-brother), was knighted for his efforts in colonizing Ireland.

I wrote a bit more about the colonial timeline of legislation [here] (https://www.reddit.com/r/AskHistorians/comments/k1f0pf/this_may_be_a_dumb_question_but_were_there_white/) and touched on the larger picture from a British perspective here, but even [Sweden] (https://www.reddit.com/r/AskHistorians/comments/l4carq/what_was_the_swedish_perspective_on_slavery_in/) tried to profit from the Atlantic Slave Trade so they all essentially followed the same course.

As that's a lot of side reading I'll end here, but please feel free to ask any questions you may still have. I'll also recommend Michael Guasco's Slaves and Englishmen: Human Bondage in the Early Modern Atlantic World Univ of Penn Press (2014) for a great glimpse of how it began to become a white vs African race system.

In addition to the excellent answer already posted When and why did slavery go from being about whoever you could enslave to being specifically about their race? written by u/groeuf may be of interest