Try this.
Get a good book on the William Miller movement concerning the return of Jesus Christ in 1843 and tell them to read it.
Miller laid out the blueprint for self-studying the bible and creating an End Times rationale backed by charts and sermons and books.
Miller convinced himself and tens of thousands of other mainstream christians of an event which never happened and radicalized their belief system. Among those he brought out of "Babylon the Great" were George Storrs and George Stetson.
Point out that George Storrs, a fallout from the Great Disappointment, and George Stetson were a huge influence on Russell.
From 1870 through 1875 the Russell family, and others, participated in an analytical study of both the Bible and the origins of Christian doctrine, creed, and tradition. "Millerite" Adventist ministers George Storrs, and George Stetson, were also closely involved. Russell's group believed they had found significant errors in common Christian belief. As a result of such study, the Russell family believed they had gained a clearer understanding of true Christianity, and were re-baptized in 1874.
Next, ask them how Russell's ideas about the 2nd coming differed (in any substantial way) from the Adventist sect that resulted from the Millerite movement.
Beliefs The doctrine of the Advent Christian Church includes belief in the Bible as the infallible rule of faith and practice, salvation available for all conditioned on repentance, faith, & faithfulness to God, and the imminent return of Jesus Christ. The doctrine of "soul-sleeping" and "conditional immortality" - We believe that death is a condition of unconsciousness to all persons, righteous and wicked; a condition which will remain unchanged until the resurrection at Christ's Second Coming, at which time the righteous will receive everlasting life while the wicked will be "punished with everlasting destruction;" suffering complete extinction of being - separates them from a number of other evangelical Christians. The church accepts two ordinances - water baptism of believers by immersion, and the Lord's supper.
Ask how the Great Disappointment of 1843 differed in any way from Russell's 1914, 1918 predictions or Rutherford's 1925 "certainty".
Miller, Storrs, Russell; three rings in one big looney circus.