The believers - the vast majority of whom were day labourers from neighbouring Egypt - prompted the Coptic Orthodox Church to declare a day to commemorate all modern-day martyrs around the world.
Archbishop Angaelos, the Coptic Archbishop of London, told Premier the men displayed a "godly peace" in footage capturing the moments before their deaths.
He drew parallels to the biblical account in Daniel 3 of a fourth figure joining Shadrach, Meshach and Abednego in King Nebuchadnezzar's furnace.
The Coptic leader said: "There must have been a 22nd [being] with those 21 men on the beach because the peace that was in their faces, the graciousness, the power, the absolute resilience could only be godly."
Scores of Coptic Christians in Egypt have died since the massacre in the Libyan city of Sirte - through subsequent attacks on the country's Christian minority.
Archbishop Angaelos said: "We've had Coptic Christians killed in church bombings and shootings in targeted attacks...
At @LambethPalace on eve of #LibyaMartyrs Commemoration to launch @MartinMosebach's #The21 through @TheBruderhof@PloughBooks. Thankful to speakers Bishop Nicolas Holman, representing @JustinWelby, @BaronessEB, #Egypt Amb Adel and Bishop Hovakim, and all those in attendance. pic.twitter.com/ay8YrZuCXN— Archbishop Angaelos ن (@BishopAngaelos) February 15, 2019
"And so, today marks the Libya martyrs but everyone since. Tragically, we also know this isn't the end; there will be others to come."
Meanwhile, a book documenting the story of the killings was launched at Lambeth Palace on Thursday evening.
'The 21' was written by Martin Mosebach. He travelled to Egypt to meet the families of the victims.
He told Premier: "These were not all at houses of mourning where I came. These people were serene, they were happy, they were proud to have a saint in heaven."
Click here to listen to Premier's Alex Williams speaking with Archbishop Angaelos:
Click here to listen to Premier's John Pantry speaking with Martin Mosebach:
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