Trinity Hurdle

Why do groups such as Christadelphians, Jehovah's Witnesses, and Unitarians have such difficulty with the doctrine of the Trinity? Do they really understand the doctrine they oppose? From the mainstream Christian perspective, perhaps a lack of understanding about the way these other groups view the Scriptures may have hampered a clear presentation of the orthodox doctrine. The Trinity Hurdle is a scriptural and historical defense of the doctrine of the Triune God and substitutionary atonement for Christadelphians, other non-Trinitarians, and those engaging with them, from an author who is familiar with both sides of the doctrinal divide.