I'm Portuguese and raised and educated (up untill HS) in Portugal and our history is quite clear on how and why we started the black slave trade and it had very little to do with religion.
We viewed ourselves as superiour to those "black savages" and because we conqured them, we could do what we pleased, including shipping thewm in boats across that atlantic.
Yes, religious people SHOULD have voiced that slavery was wrong a lot louder than they did, but the question of slavery was far more than a moral one at the time, it was a social-economical one and only later did the moral wrongness of slavery OVERCOME the economic rightness of slavery.
And, if I recall correctly, it was a "christian moral outrage" that drove to abolish slavery.