My Musings – If: an introduction to a conditional clause. One can only be a new creation if they are in Christ (the condition). If one is not in Christ, they are not a new creation, In: a specified relationship, as in a relationship with (saved by) Christ. Is: the present tense of being. If one is in Christ they will not become a new creation, they are a new creation.
The old (life dominated by our sin nature) has gone (no longer here) and the new (life dominated our Son nature) has come (is now here). This is what it means to be a new creation.
My Advice – Our old life can only master us if we choose to let it. Because we are a new creation, we have the ability to “just say no” to our old nature and habits. Choose wisely.