... what could be more serious than one being rejected by Jesus for ever, because that one has never really known the true Lord Jesus Christ of Holy Scripture at any time ever at all during their whole life?
Do we, these days live, as foretold, in the midst of a wicked apostate adulterous people who delight to call 'Lord Lord' ... but do not care to let Him really be Lord?
... recall that the Lord Himself will send strong delusion ... so that those who (persist to) take pleasure in unrighteousness (after being often rebuked) will believe a lie as if it were the truth ... but instead will come into crisis ...
Recalling the words of Rev 20:4-5 ... we best get ready for great tribulation and be prepared to die if that is what our Lord wills for us.
In the meantime, we best call all, ourselves included, to repentance and loving trust and obedience to the Lord Jesus Christ.