Peter is stirring up Christians to know who they are in Christ & helping them recognize their new identity. After all, when one is born again, a new, reformed version of you is in development. Peter mentions to them to crave God's Word as a baby craves milk. Milk helps you grow in development and in this analogy, if you're not "drinking the milk", taking in God's Word, and cultivating your relationship with God, you are not going to grow. Peter also illustrates how things such as malice, deceit, hypocrisy, slander, etc. are all regular characteristics of a worldly person. Peter calls out Christians who still do these things. Peter advises them to "take them off" like dirty, filthy, smelly clothes, grow up, and mature. Then from there, you can go from "milk" to "solid food".