Even someone who passes that test can have faulty doctrine, as you have stated. That’s why it needs to line up with Scripture or have Scriptural support.
Just because someone is truly a Christian, doesn’t mean I believe everything he claims to be from God.
Apollos had to be better instructed by an interested Christian couple.
This world is entering its darkest phases and scripture has stated that Judgment first starts in the House of God. Should Christ still tarry for a while, God will empower his believers who will stand true and above all stand. There will be many voices claiming to speak for God and we’ll need to know how to test them...some of them will speak truly and will pass the test of 100 per cent accuracy and will line up with scripture as God has instructed us in the old testament. The rest will be false and of the spirit of Antichrist. At some time during all this, the rapture comes...”therfore comfort each other with these words”.