One aspect of theistic evolution that you need to reject is the view that there was no initial human or pair of humans such as Adam and Eve. Why is that important? That is important because the Old Testament and the New Testament put a lot of theological weight on the reality of Adam. For instance, Romans 5 talks about how Adam sinned and how that sin was imputed or legally given to all humans. In addition to that, if you come from Adam, you’re born with the tendency to sin. All of that comes from one human.All those connected to Adam are sinners, but all those connected to Christ are righteous.
In Romans 5, Paul talks about Adam and how that one man created all these sin problems. He says that God purposely had another human, the Lord, Jesus Christ, who was both fully human and fully divine. From him, all good came. Paul sets up the parable: from Adam, all sin came; from Jesus, the second Adam, the last the Adam, the God man, all good came. All those connected to Adam are sinners, but all those connected to Christ are righteous because of the work that Christ has done.
One way of answering, “What is a Christian?” is to say that it is one who moved from Adam as his father to Christ as his Father.