1 John 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
This is yet another verse that confirms that those are “are of us” would “have continued with us”.
Those who are in the Kingdom of God will surely continue in the Kingdom of God, and not fall away.
God allows apostates in the church to train godly saints in discernment (for then they have a better grasp of scriptural truth), and also to reveal the hearts of members who are not saints (for they are the ones who embrace and stand with false doctrine and immoral conduct).
Bibloscope 