Dictionary of Biblical Words:
This word is also translated “creature” and the expression, “a new creature,” occurs 2 Cor. 5:17 and Gal. 6:15, and describes the absolute newness and divine origin of the life and nature of the believer. With the definite article, however, the expression is used more generally as descriptive of the new work of God beginning with the resurrection of Christ, the beginning of the creation of God (Rev. 3:14), passing on to ourselves, and ending with the new heavens and earth. In this way it is an interesting parallel to Gen. 1. Christians are spiritually in the new creation, though their bodies and earthly ties and duties are in the old.
Concise Bible Dictionary:
This stands in contrast to the first creation ranged under Adam, who was blessed by God, and should have maintained his allegiance to Him “If anyone be in Christ, there is a new creation; the old things have passed away; behold all things have become new; and all things are of God.” “If even we have known Christ according to flesh, yet now we know Him thus no longer” (2 Cor. 5:16-18). Those who have died with Christ, and have risen with Christ, have lost their standing in the first Adam, and are in the Second man. “In Christ Jesus neither is circumcision anything, nor uncircumcision; but new creation. And as many as walk by this rule, peace upon them and mercy, and upon the Israel of God” (Gal. 6:15-16). This is the wholly new position into which the believer is brought in Christ. Still, while in the body he is not entirely free from contact with the old creation: the wilderness life is a part of Christian life, as well as Canaan and its conflicts. In reverse order to the first creation, here the Man was first brought out (Christ risen), and then those that are His, and lastly the heaven and earth (Rev. 21:1).