The Constitution refers to Christianity and Jesus.
Accommodationists and others opposed to the separation of church and state sometimes argue that government support and defense of Christianity is justified because the American Constitution refers to Christianity: in Article VII, the Constitution is dated with the words "the Year of our Lord one thousand seven hundred and Eighty seven." What relevance does this point have for the debate over religious liberty? Absolutely none. This was simply the dating convention, not an ideological statement.
Yes, the authors and signers of the Constitution relied upon a dating system which marks as its beginning the birth of the founder of Christianity, Jesus Christ. Yes, it was custom at the time of the Constitution to set forth dates by writing them out in long hand and to use the phrase "the Year of our Lord." It would have been odd for the document to be dated any other way.
None of this would allow us to conclude that any or all of the authors and signers were Christian (though they were), much less that they considered Jesus Christ their "Lord" or that they regarded him as somehow the sovereign over the government. Quite the opposite, in fact: the Constitution is designed to ensure the sovereignty of the people, not of any religious figure.
Moreover, if the use of this phrase was designed to communicate a favored status for Christianity, why use such an obscure method coming at the very end of the document? Had the authors wished to establish Christianity as a partner with the American government, or even to send the message that Christianity occupied some foundation or inspirational role for the government, they could have done so much more explicitly and in dozens of more substantive ways. However, they did not — and that silence speaks very loudly.
Days, Months, Years
In addition, if a Christian really wants to argue that the use of Christian dating implies a Christian basis to the government, they're in a lot of trouble because the names of the months and days have pagan rather than Christian origins. Although even conservative Christians today don't give this a second thought, it was evidently a source of some consternation of Quakers who refused to use the pagan-based names.
Nevertheless, the authors of the Constitution refused to employ the Quaker numerical system and chose instead to stick with the standard pagan names for days and months — just as they used the standard Christian way of marking the year. This suggests that the authors really didn't read much into such naming conventions, effectively undermining the argument that the phrase "Year of our Lord" has any significance worth discussing.
Ultimately, anyone using a dating method to argue against church/state separation generally or against the secular nature of the Constitution in particular is engaged in a transparently desperate exercise. It's the sort of argument that a person reaches for only after absolutely every other option has been shut down or if they are so completely ignorant that they just can't recognize the difference between reasonable and unreasonable arguments.