func authenticateUser()
{
    let user = FIRAuth.auth()?.currentUser
    var credential: FIRAuthCredential

    //prompt user to re-enter info

    user?.reauthenticateWithCredential(credential, completion: { (error) in
        if error != nil
        {
            self.displayAlertMessage("Error reauthenticating user")
        }
        else
        {
            //user reauthenticated successfully
        }
    })
}

let credential = FIREmailPasswordAuthProvider.credentialWithEmail(email, password: password)

let credential = FIRFacebookAuthProvider.credentialWithAccessToken(FBSDKAccessToken.currentAccessToken().tokenString)

let credential = FIRTwitterAuthProvider.credentialWithToken(session.authToken, secret: session.authTokenSecret)

let authentication = user.authentication
let credential = FIRGoogleAuthProvider.credentialWithIDToken(authentication.idToken, accessToken: authentication.accessToken)

let eMail = EmailAuthProvider.credential(withEmail: "some@email.com", password: "somepassword")
    let fb = FacebookAuthProvider.credential(withAccessToken: "xxx")
    let g = GoogleAuthProvider.credential(withIDToken: "xxx", accessToken: "xxx")
    ...

    Auth.auth().currentUser?.reauthenticate(with: eMail, completion: {
        [weak self]
        (error) in
        ...
    })