Church deacon Daniel Irwin testified he peered through a doorway window in the sanctuary during the more than 12-hour ordeal at the Word of Life Christian Church, New York, and saw Lucas Leonard bleeding and in apparent agony.
Within hours, the young man would be dead, killed by blows inflicted by his parents, sister and fellow church members, authorities said.
The testimony came at a court hearing for the parents, Bruce and Deborah Leonard, on manslaughter charges.
At the conclusion of the hearing, a judge ruled there was sufficient evidence to sustain the charges.
The arrests in New Hartford, a town of 22,000 people, have thrown a spotlight on Word of Life, a highly regimented and insular church that operated out of a large, red-brick former school that also served as a communal home for several members.
Police Chief Michael Inserra said outside court that members told authorities Lucas Leonard and his 17-year-old brother, Christopher Leonard, were beaten during what began as spiritual counseling Sunday night over Lucas' desire to leave the church.
A neighbor, James Constantine, also said Lucas had talked about moving on and had mentioned he might join the Army.
Christopher was hospitalized in serious condition, but his health was improving.
Four other church members, including the victims' 33-year-old sister, Sarah Ferguson, have been charged with assault.
Their hearings will be held later.
All six defendants have pleaded not guilty.
