Executed right-wing terrorist Tim McVey is often brought up as a counterpoint to the 9/11 hijackers, particularly in response to irrational conflation of those hijackers with "all of Islam." While the comparison is imperfect, the right has frequently responded with posts claiming McVeigh was an atheist. But the real argument should not be that McVeigh was a "Christian." That term is too subjective to provide clarity. What is objective truth is McVeigh's affiliation with the Christian Identity movement. Like most terrorists, McVeigh played religion rather than practicing it. Neither he nor the 9/11 hijackers likely possessed the intelligence to think through that distinction, but religious terrorists are classified as such because they deploy religion, not because they practice it.
One needn't be a "Christian" to be a "Christian Terrorist." The majority of American Muslims did not and do not see the 9/11 hijackers as legitimate Muslims. It's certainly not clear he was an atheist. When interviewed by Time, McVeigh claimed that although he had "lost touch" with Catholicism, he still "maintain(ed)" its "core beliefs." He told Time " I do believe in a God..." and declined further religious discussion. He wrote a letter to the Guardian claiming to be an agnostic, but accepted Last Rites before his execution. More important is what influenced McVeigh: The Christian Identity movement and its eschatological catechism, The Turner Diaries.
McVeigh self-identified with Christian extremism. Whether one is a full-on professed member of a religion is not as important as whether they are willing to (a) model their behavior along the ideological lines of that religion and (b) exploit others' religious beliefs to their own ends. McVeigh made several calls to the Elohim City enclave in the weeks prior to bombing the Murrah Building. McVeigh had also signed up for membership in the Ku Klux Klan, an organization that specifically requires that its members "conduct themselves with Christian character."
All of which is simply to say: McVeigh was to Christianity as the 9/11 hijackers were to Islam.