If we cannot agree on the definition of Christian, then how can we begin to answer the OP?
Perhaps, for the sake of discussion, we could forget some all-encompassing definition and instead agree on a general set of parameters based on faith (
x) and practice (
y) within which Christianity is generally found.
https://s16.postimg.org/maqqvbhed/image.png
We might say the Nicene Creed, agreed upon by the majority of the Christian world for millenia because of its clear proclamation of the Triune God (Father, Son and Holy Spirit), of Christ’s saving work, and of His church, forms this center of belief - appropriately symbolized in the chart above by Him Crucified.
We could call this intersection of
x and
y, which is described in the Nicene Creed,
orthodoxy. Those who are closest to
x,
y are within orthodoxy and as the CCC states, “have been justified by faith in Baptism [and] are incorporated into Christ; they therefore have a right to be called Christians,” despite holding slightly different views.
Those who hold generally orthodox beliefs but interject novel practices or reject some traditional practices might appear in the correct place according to
x, but would be higher or lower on the
y axis. We could call these
unorthodox, but still acknowledge them to be essentially Christian.
Those who hold to generally orthodox practices but interject non-Christian beliefs or reject some Christian truths might appear in the correct place with relation to
y, but would be left or right of the norm with regard to
x. We might call these
heterodox, and still acknowledge them to be Christian – provided they did not stray so far on the axis as to lose the faith entirely.
Stray too far on that
x axis, and a group could leave heterodoxy altogether and find itself in
heresy, at which point it would cease to be Christian, despite any outward appearances on the
y axis.
Those groups that have abandoned not only orthodoxy of
y, but also of
x, are no longer on this chart. They belong on line
z, which is non-Christian.