Was Christianity the cause of slavery in the US?

Christianity was not the cause of slavery in the United States. However, it was used to justify slavery.
Greed and the drive for profit were the cause of slavery. Landowners in the colonies realized they could make a huge amount of money growing sugar, tobacco, and later cotton if they had slave labor to plant, tend, and harvest the crops. In fact, the work was so harsh that paid workers couldn't be relied on to come to work everyday—or wanted to be paid so much it cut into profits. Therefore, landowners purchased slaves to do the brutal labor that made the owners rich.
Slavery was a profit center up and down the line. Tribes in Africa earned money (and got rid of rivals) by rounding up people to sell to slave traders. Slave traders made huge profits selling the slaves. Plantation owners made huge profits on slave labor. Slave owners could also make money breeding and selling slaves. The problem was for the slaves, who suffered immensely under this sadistically cruel system.
US slavery was so obviously inhumane and immoral that there was an outcry against it. A cruel system need an ideological justification that helps cover up and confuse people about its inhumanity. For many slave owners, the Bible, with Old Testament stories about Biblical patriarchs owning slaves and New Testament verses about slaves obeying their masters, seemed to say that God sanctioned slavery. Slave owners were not shy about repeating these verses to justify an immoral system. So, while Christianity did not cause slavery, it was used to perpetuate it. Many other Christians, of course, used Jesus's teachings to fight for the abolition of slavery.