That’s a tough question.
Technically, Christian refers to a follower of Jesus Christ.
A validly baptized person may apostatize- become an unbeliever, either to another faith, or a life that’s incompatible with Christian teachings, or indifferentism. They could not then really be said to be followers of Christ even with their valid Trinitarian baptism, could they?
And yet not all lapses of faith and practice, not even all serious ones, constitute apostasy

.
On the other hand, a
believer and
follower of Christ, while still a Catechumen (preparing for baptism) cannot also be said to
not be a Christian.
Yet Baptism
makes you a Christian… As does faith…

It’s difficult.
My own personal opinion: A
believer in
Christ and
the Trinity who is doing or genuinely intending to do (even with failures) all that is required of him in Christianity or is at least aware and
acknowledges in belief, his duty to do so, whether/not he actually does it.