I suppose if 'sheep' and 'goat' were figurative you'd have a point. But as it is, it is figurative, not literal. We don't become literal sheep when we get saved that somehow we can't now be changed into a goat.
Yes,
THEY were not of them. If this meant
everyone who goes out was never really saved to begin with then John could not tell the believers who remain to let the truth remain in them.
19They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.
24As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father."-1 John 2:19,24
To be saved, and to continue to be saved, you must let the word by which you were saved remain in you. That is how you continue in the Father and the Son in salvation.
Besides, what a horrible doctrine of security once saved always saved perseverance is. You can't know that you are truly believing and are saved until your life is over and you have exhausted all opportunities to leave and expose yourself as being one who never really believed. And that's supposed to be the doctrine of peace and security and assurance????
But in simple Biblical security, you know that you are indeed fully saved and ready to meet Christ when he returns by if you are presently believing. And you know you that your belief is genuine by your changing life. That is the Bible version of security and assurance. Keep believing, don't let the word of the gospel slip from your heart. You will continue to abide in the Father and the Son if you do that.