4964 sysxēmatízō (from 4862/sýn, “identified with” and sxēmatizō, “possessing outward shape”) – properly, to assume similar outward form (expression) in following the same pattern (model, mold).  

4964/sysxēmatizō (“having the same shape, form”) occurs twice in the NT (Ro 12:2; 1 Pet 1:14), expressing the outward impact of sin on people living without faith.  Here human nature here conforms to the “shape” (“mold”) of this world (missing God’s mark).  

Reflection: Our outward identity (form) always reflects our “inner man” – either based on self (self-will, the flesh) or conformity to Christ through His inworking of faith.  Note the force of the prefix (4862/sýn).

Near-synonym

4833 (symmorphoō) refers to “conforming in one’s inner nature” (S. Zodhiates, Dict).  4964 (sysxēmatizō) focuses on the outward form sin takes on, i.e. exterior expression of being self-willed.

Commentary

(1 Pet 1:14)  not fashioning (conforming) yourselves (4964/sysxēmatízō) – This Greek verb is either: a) passive voice = “Do not allow yourselves to be conformed”; or b) middle voice = “Do not conform yourselves,” i.e. “For your own sake, do not be conformed . . . “.  

Both senses may apply at the same time.