We have finite minds with limited knowledge and therefore, when we study God's word, we are approaching the infinite One who has all knowledge with limited thought processes.
Why don't you just become an agnostic then and let it go? Are you serious? Is the Word of God so unclear, uncertain, ambiguous on the most important doctrines in scripture and especially the most important doctrine, which is called the truth of the gospel? That is how Paul describes justification by grace through faith without works - the "truth of the gospel." If what you say is the case then someone had better tell that to Paul who said that all who preach justification by works are preaching "another gospel" and are to be regarded as "accursed". However, if our poor finite brains are insufficient to clearly understand what Paul calls "the truth" of the gospel, and those who deny that truth are simply trying the best they can to ascertain the truth and walk in it just like Paul, then Paul better cut back on his rhetoric as he is being too harsh against sincere people who differ with him.
To say those who oppose eternal security are cults is rather harsh. They just have come to those conclusions the same way we have, via studying.
The Trinity is a hard doctrine too? So is the virgin birth? So is the doctrine of Christ's person and there are sincere devout professed Christians that oppose them and have opposed such teachings throughout history. Do you really understand what you are saying???
When a person denies (not talking about being ignorant and neutral on the issue) eternal security they are claiming in the clearest possible sense that Jesus Christ and his own personal obedience to God was insufficient to justify them on judgement day APART FROM works of obedience performed in and through their own bodies. It makes no difference if they attribute their faithfulness to God's grace or not, as they are repudiating the obedience of Christ as sufficient in and of itself to justify them. It does not matter if they explain that their own works were empowered by the Spirit or the products of grace as they are repudiating the sufficiency of Christ's works in his own body to satisfy God's complete demands against them. It does not matter if they explain that their own works performed in their own bodies by the Spirit of God is simply the necessary consequence of regeneration if they are including it as the grounds for justification before God. It does not matter how they sugar coat it, explain it, defend it, because it boils down to the same conclusion and that is the complete repudiation that the personal obedience of Christ in his own body alone is sufficient for ultimate justification. It is the denial that we are justified before God by "imputed" or alien righteousness, but rather we are justified by imparted righteousness in and through our own person. It is a denial that God justifies the "ungodly" but only the "godly." It is the assertion that justification is sanctification and that is the whole basis of Roman Catholic soteriology and the essence of "another gospel."
We will have a lot of things shown to us that we believed as truth that were actually false, yet God will grant us entrance anyways.
If our experience matches our belief in justification by faithfulness to Christ we are lost and going to hell - period! If we are saved and have been led astray on this doctrine we are acting as enemies of Christ whenever we deny this truth - period! There is no middle ground.
Certainly no Christian knows all truth, but they must know the essentials of the gospel to be a true Christian. If God has not REVEALED Christ in their heart as the all sufficient Savior then they are still lost as the very essence of regeneration is the revelation of "the light of KNOWLEDGE of the glory of God in the face of Jesus Christ" within the heart (2 Cor. 4:6) as "this is eternal life to know the only true God and Jesus Christ (Jn. 17:3).
We are living in the greatest period of apostasy and confusion the world has ever seen and that is precisely what Christ predicted before his coming (Lk. 18:8).
If we followed your thinking it would lead to further apostasy as you are willing to tolerate any doctrinal falsehood as long as that person sincerely believes their error and professes Christ as their savior, simply because we are all finite and attempting to defend our position by the scriptures the best way we can.