Christians who suffer torture and murder by ISIS are like the first century Christians who were "besieged by persecution", according to the Pope.

Describing violent atrocities carried out by Herod who put James, brother of John to the sword and seized and jailed Peter, Pope Francis said "terrible, inhuman and inexplicable persecution" is unfortunately still present in many parts of the world.

Speaking on the feast of the apostles St Peter and St Paul, he said this persecution even today takes place often "under the eyes and in the silence of all".

He said the courage of the apostles and the first Christian community in continuing the work of evangelisation without fear of death and martyrdom in a pagan empire was "a strong call to prayer, faith and witness" for believers today.

In his homily the Pope said: "How many forces in the course of history have tried, and still do, to destroy the Church, from without as well as within, but they themselves are destroyed and the Church remains alive and fruitful."

He said Christians were duty-bound to evangelise. "A church or a Christian who does not give witness is sterile, like a dead person who thinks they are alive, like a dried up tree that produces no fruit an empty well that offers no water."

And the strongest weapon for the Christian was prayer. "A Christian who prays is a Christian protected, preserved and supported, but especially not alone," he said. "Prayer is the encounter with God, with God who never lets us down, with God who is faithful to His word, with God who does not abandon His children."

The forces of evil such as ISIS will not prevail, he predicted.

"How many forces, throughout history, have tried - and try - to destroy the Church, both from outside and from the inside, but they are all destroyed and the Church remains alive and fertile!" This was because "the Church is not of the Popes, bishops, priests and even of the faithful, it is only Christ. Only those who live in Christ's Church promote and defend the sanctity of life, the example of Peter and Paul."

There is no force capable of defeating those with the power of faith, he said.