Example for simple authentication using RNFirebase

App.js

// ...
// Inside our Component's class

_authenticateUser = async () => {
  const { firebase } = this.props; // Parent passes reference to Firebase through props
  const { userPassword, userEmail } = this.state;
  
  let userInfo = null;
  try {
   userInfo = await firebase.auth().signInWithEmailAndPassword(userEmail, userPassword) 
  } catch(error) {
    // Error will contain information like Invalid credentials or User does not exist
    return this.setState(() => ({
      authError: error.toString(),
      isFetchingAuth: false,
      isAuthenticated: false
    }))
  }
  // We're authenticated!
  // We don't need to store userInfo, just check if value is not null.
  this.setState(() => ({
    isFetchingAuth: false,
    isAuthenticated: !!userInfo
  }))

}