Paul Barber's speaking after the government announced in September it would be relaxing a cap on faith schools, which forced them to selectively choose no more than half of their pupils
In the future, faith schools will be able to selectively admit all of their pupils on the basis of their faith.
The government says the previous cap was not working, but critics say removing it will create so-called religious ghettos.
At the time Paul Barber told Premier he supported the plans, saying it would allow the Catholic Education Service to open more schools across the country.
It was previously disincentivised from doing so because of the cap, which would mean if it opened new schools it'd have to turn away hundreds of Catholic children each time.
Speaking to The Times Higher Education Supplement, he said: "The move back to schools of 100 per cent one faith is dreadful.
"We're very clear about the nature of the education that we're offering: an unapologetically Catholic education.
"If others want to be part of that, then we want to have the spaces to be able to welcome them. That's a real blessing for the pupils: to have children of other faiths in our schools."
A Downing Street spokesman said in September: "The admissions cap had the best of intentions but it has failed in its two key tests. It has failed to make minority faith schools more diverse, because parents of other religions and none do not send their children to those schools.
"But it has prevented new Catholic schools from opening, which are more successful, more popular and more ethnically diverse than other types of state school.
"We're going to change the rule, so we can allow new Catholic schools to open, while making faith schools of all kinds do more to make sure their pupils integrate with children of other backgrounds."