Antonio Campos, 26, was stabbed in the stomach with a seven-inch kitchen knife after he was surrounded by more than 20 teenagers.
The Brazilian student was in a group of six evangelists in a car on their way to central London to minister to the homeless, when their leader, Pastor Derek Wilson, saw the youngsters 'squaring up' for a fight.
Detective Inspector John Beeston said that Pastor Wilson and several other members of his congregation went to intervene.
'They did manage to stop whatever was going on, but the victim got split off and was surrounded, up against a wall, by about 20 or 25 youths,' Inspector Beeston added.
'They were only young teenagers, 14 or 15 years old.'
'They were shouting at him, but he understood little English and didn't talk back. Then one of them came out of the crowd and stabbed him.' The attack happened at around 10.30pm on Friday at Shepherd's Bush Green, west London. He died from his injuries in Hammersmith Hospital on Sunday.
DI Beeston said: 'They were all part of the same evangelical church . . . based in Latimer Road, Notting Hill, west London.
'The victim was a member of the church, not in the least the sort of person who would provoke trouble. They were only trying to be Good Samaritans.'
Mr Campos had travelled from Brazil to London to study and was living in north London.