So perhaps the difference is a philological one. But the idea that lies behind the phrase is an important one.
Are you a moralistic Christian or a Christian Moralist?
The second word, i.e. the proper noun, is the primary designation of the person, and the first word is its adjective.
Simply put, are you a Christian who is moralistic, or are you a Moralist who happens to be Christian.
The key difference lies in which comes first. For the church, in a loose sense of the word, today is full of Moralists, who see religion as a matter of do’s and dont’s and happen to identify with the Christian code of laws, which they believe to be the main revelation of the Bible.
But the Bible calls us first and foremost to be followers of Christ, i.e. Christians. Followers in the sense of sinners who trust in Christ to pay the penalty of their sin and become their righteousness, nailing their old sinful selves to the cross and putting on their new selves, created to be like God in true righteousness and holiness. It is in this light that they adhere to a new law. The law of Christ.
Moralistic Christians are firstly Christian, repentant and saved and made righteous and reconciled to the Father by the death of Jesus. Then they are moralistic for they are now subject to a new law, the law of righteousness, which leads to eternal life.
In the strict sense of the label ‘Christian’, all Christians are moralistic. But all Moralists are not Christian.
Which one are you?