Bethea's funeral was held at Saint Mary's Roman Catholic Church in Bordentown, New Jersey.
He tragically lost his life earlier this month during training.
Bethea, 33, was described as completely selfless and always in service of others. He left behind a wife and a young daughter.
"I'm forever grateful for every moment we had together," said his wife, Kate Bethea, as she delivered the eulogy.
On May 5, while training to become a member of Teams Unit One (the NJSP SWAT team), the eight-year veteran of the force died at the barracks in Ewing Township, Mercer County.
"I'll never forget the way his eyes would light when he talked about being a state trooper, especially becoming a Teams member," Kate Bethea said.
He was a graduate of Northern Burlington Regional High School, where he participated in Trooper Youth Week and went on to earn a bachelor's degree at Rowan University.
Fellow trooper and childhood friend, Tpr. Samuel Leibman described Bethea as "Superman."
"Dirtbike riding, fast cars, shooting. If it was manly, he probably did it. On the flip side, no one enjoyed a good sit more than him, except maybe your grandfather," Leibman said.
New Jersey Governor Phil Murphy spoke at the service as well.
For a short time, Bethea protected Gov. Murphy and First Lady Tammy as part of the executive security detail.
"We are eternally grateful for Marcellus and his lifetime of service and sacrifice. The truth is, by any measure, Trooper Bethea was a hero," Gov. Murphy said.
Bethea suddenly died during exercise training. It happened on May 5 at New Jersey State Police headquarters in Ewing Township, Mercer County.
Gov. Murphy ordered all flags in New Jersey to be flown at half-staff on Wednesday in honor of Bethea.
Authorities have not yet released how Bethea died.
May 15 is nationally recognized as Peace Officers Memorial Day. The day encourages Americans to honor those who have died in the line of duty.