Like you, I’ve seen signs like this posted by relatively conservative groups and individuals on social media and elsewhere. While I understand the impetus behind such signs, that I understand does not imply that I agree with them – far from it!
For one thing, we must remember that our President IS a Christian. Until shortly before his election to the Presidency, he and his entire family regularly attended worship at Trinity United Church of Christ in Chicago, one of the largest and most dynamic churches within my own denomination. In fact, more than a quarter of a century ago, our future President made a conscious decision to become a Christian when he joined that church after being, as he put it, a “religious skeptic” for many years. (One should also note that Trinity Church’s motto is “Unashamedly Black and Unapologetically Christian.” …And as an organization serving a community deeply and continuously affected by racism and injustice in this society, how can their motto be otherwise?)
Now, I am a Minister in the tradition of the United Church of Christ (UCC), and for over three centuries members of my family and our ancestors have been faithful members of a church that is now part of this same Protestant Denomination that Barack Obama chose to become a member of at age 26. Obama is one of three presidents who were affiliated with this same religious tradition that the UCC descends from (Congregationalism) at some point in their lives. So, I see signs such as this being evidence that their bearer does not believe I am a Christian, either.
The Here and the Hereafter 