Austen Ivereigh, from Catholic Voices, the Church's mouthpiece in the UK, said security was "absolutely front and centre to the extent to the extent that it's really dominating everything".
NYPD Commissioner Bill Bratton said it was "going to be an unprecedented challenge for this department that has dealt with many precedents in the past".
Pope Francis is due to land in the States on 22 September for a six day visit that will include a stay in New York, Washington DC and Philadelphia.
Austen Ivereigh, who is also author of Pope Francis biography The Great Reformer, said: "Security is absolutely massive for this and I think security has been a far bigger driver this time than it has been on previously papal visits because of the threat from ISIS [Islamic State].
"They've made very clear they are planning, and would love, to attack the Pope.
"And because of the legacy of 9/11."
He added that there was only one major mass during the trip.
Pope Francis will address the United States Congress, meet with President Obama at the White House and visit a multi-religious service at 9/11 Memorial and Museum.
