It’s an oversimplification. Much of Christianity has much in common, but there are sincere differences and sincere difficulties as well.
As evidence for this we could list just a single example: the
joint declaration by Lutherans and
Catholics on justification.
While it’s true that this document might outline certain similarities between two theological schools in Christianity, it’s also true that the Catholic Church offered
an official response to this document, which itself outlines certain impediments and clarifications which nonetheless continue to exist and which must be made in order to avoid confusion.
Examples of similarities and differences amongst Christian theologians could be multiplied.
Also, “basically the same” implies that there are certain doctrines which are essential to Christianity, and which distinguish the Christian faith from other religions. But how do we know which doctrines are essential and which aren’t? Who gets to decide and why do they have that authority?
I would use such questions to point a person in the direction of the Magisterium and the Papacy, using common apologetics along the way.