A 15-year-old boy has been arrested after allegedly stabbing a bishop and a father at an Orthodox Christian church in Sydney's west.

The attack at the Christ The Good Shepherd Church in Wakeley sparked tremendous outrage. By the time police arrived, they were forced to bunker inside the building to protect the arrested teenager.

Bishop Mar Mari Emmanuel was giving a sermon, which was being livestreamed on the internet, when the 15-year-old approached and allegedly stabbed him multiple times just after 7pm on Monday.

Father Isaac Royel was also stabbed, police said. Both Emmanuel and Royel were taken to hospital with injuries that were not life threatening.

Parishioners launched on the teenager, and apprehended him until police arrived, acting assistant commissioner Andrew Holland said in a press conference.

The bishop had protested Israel's war against the Palestinian people in Gaza.