Spencer makes the argument that early Muslims were the rejected followers of the Christian heretic Arias.
The Arias cult had spread throughout North Africa and the Arabian peninsula by the 6th century.
The rapid conversion of Christians of those areas to Islam cannot easily be explained by the sword... Islam, once it reached non-Arian parts of the world, shed much more blood converting much smaller areas.
But for Arians, who denied the divinity of Christ, to convert to Islam, which denies the divinity of Christ, is much easier to swallow.