I think it is extremely unlikely if not evidently impossible that there is an argument in existence that can prove to me that slavery conforms to the nature of love.
Agreed. Because love by definition is the willing of the good of the other person, not the willing of your own good. But "pure" slavery is when one human being is converted into the good of another without any reference to what is good for the one subjected. So what belongs to the subjected belongs to the master--from the product of his labor to his body to his life.
So when one man's very life is unconditionally subject to the absolute will of another man--when you have "pure" slavery--it has nothing to do with love and is thus against the Gospel of love. It is rank injustice on behalf of selfishness, under cover of lies.
.... black people....were considered as sub human creatures with enough intelligence to use tools. This kind of slavery is relatively recent; and slavery is still happening to day in some parts of India and other places, if I am not mistaken. The early Jews in the days of Moses were themselves slaves.
Yes, the black people in the States before the Civil War were subjected to the absolute form of slavery, as were the Jews in Egypt--that is, the master could impose anything including death at will without any restraints whatsoever. Man's inhumanity to man. The mind-boggling injustice, not to mention cruelty, of this slavery recalls other mass enslavements and slaughters in modern times--Stalin killed millions, Mao also, millions killed in Cambodia; not to mentions Hitler's slaughter, and not to forget our own slaughter since Roe v Wade of 45+ million of our own unborn children, treated as absolute property.
Without Christ there is no hope for fallen man; no escape from our cruelty to each other. So we look to Him and to His Church.
I am very glad you found the 1435 bull Sicut Dudum. Before this new tide of absolute slavery for the New World began to be commited upon the African peoples, the Catholic Church was already on record against such practices--which had vanished from the areas reached by Her authority. People forget to evaluate an event within its historical context. Research shows that the Church walked Europe out of slavery over time--that is, until Protestantism exterminated America's native peoples and enslaved other peoples to build wealth in the new world.
Please note a major generalization: Why do we have what we call Mexicans today? Because, despite the bloody battles, the incoming Spanish then married the native peoples (after destroying the devil-worship). They neither exterminated nor enslaved them--I am speaking generally here. Meanwhile the Protestant Europeans did their best to wipe out the natives farther north, eventually corralling the remnants. People do not give the Catholic Church any credit, when She did in fact succeed in protecting from and putting down slavery in the lands where she had some power. It is North America which is infamous for black slavery--not South America. It is not Catholicism which settled North America, but Protestantism--let us note.
I believe the Christians in Sudan are being forced into slavery by their Islamic neighbors to this day.
They say that saint Paul said that the slave should remain with the slave owner.
We read this as "remain as Christian brother with Christian brother." St. Paul also said, "be ye subject to one another," why don't we include this quote to give context for the other quote? Saying "be subject to each other" in the context of Paul's times was revolutionary. You cannot overturn such a culture-wide financial mechanism overnight however wicked; but you can soften and discourage it such that it is on the road to extinction, as Abraham Lincoln said was a good strategy in the face of such an entrenched thing.
There are some-things I am willing to take faith on. But my faith stretches only so far.
I am not clear how this matter of slavery is a matter of faith. I think the Church has never in 2,000 years taught it was good to enslave anybody even conditionally. It is in Her teachings that we must have faith. I do know that She repeatedly acted to modify and moderate the practice (such as your Bull), and that under Her growing authority Europe emerged from the Dark Ages into the Middle Ages--and out of slavery into serfdom tending toward freedmen with large landlords. How to explain this disappearance of slavery from Europe? (Until the so-called "Enlightenment" when classical Roman values including Roman slavery re-awakened in Europe and seemed to give the newly Protestant peoples license to enslave). (I'm not saying no individual Catholic was involved in the slave trade--I'm saying it was Protestant America which formed the market for African slaves, the market being "stocked" by bad "Christians" some of whom were Catholics disobedient to their Church.)
We should notice and beware when the Church is criticized for historical events--she was/is both weaker and greater than her critics understand.
Would some reading in history be helpful, as an antidote to the slanted "history" popularly taken for granted today?
Belloc's The Servile State is an easy read, and very illuminating. Those Terrible Middle Ages by Pernoud is entertaining in its peevishness and revolutionary if one has never heard the other side.
To go deep into history is to cease to be Protestant--I believe Cardinal Newman said that. May I say, do not fear to research this matter as much as you have to time to do--it will vindicate the Church, giving you all the ammunition you need to defend Her to your family and to stay staunch yourself.