> A lot of Christian growth came from enslaved or exiled Christians. However, once adopted by the king, then it became more top down.
Once Christianity became the state religion of the Roman Empire in the 4th C, its spread was largely by conquest and top-down, both within Europe and through European colonialism.
> Even after the Roman Empire collapsed in that same century?
It didn't. It split shortly after, and the post-split Western Empire collapsed nearly a century later, the Eastern Empire about a millenium after that, and both had top-down imposed Christianity throughout their domains before falling, as did many of their successors.
How can you explain European Christianity with top down imposed rule if the Western Empire collapsed before the medieval ages? What is doing the imposing?
Once Christianity became the state religion of the Roman Empire in the 4th C, its spread was largely by conquest and top-down, both within Europe and through European colonialism.