Considering the letters of St. Paul, St. Peter, St. John and St. Jude, and the contents of the Book of Revelation, it seems apostacy was widespread towards the end of the 1st century, and so there should not be any problem in stating that the ones using the catacombs in fact were apostates. OK, they did not use the Cross, but they had sets of beliefs which included the reverence towards bishops, the belief in the soul etc.
This, however, gives birth to what in my eyes is another, deeper, problem, which is scarcely debated at all: How did the congregations and groups react to the letters of the said writers? Given there was this great apostacy around the end of the 1st century, obviously the letters and admonitions had no impact at all - the ones written to did not care, but kept on in their apostacy.
Leading to the final question, how come that the teachings of Jesus Christ, that Christianity, was such a weak religion? Why was it that it only had a true impact on people for a handfull of decades, but then fell victim of an apostacy, which has blurred it ever since?
IF that is what happened?