What is the connection between "outer darkness" and "gnashing of teeth"?

Why is the Bible so vague, meaning why can't most Christians or so called Christians understand the dispensation of Paul's message? The difference between the Gospel of the Kingdom and the Gospel of Grace.

The Bible states that God "predestines" Christians for a future with Christ in Heaven and that he does NOT predestine others to Hell; however, does this mean that only a few of the Christians on Earth were predestined by God and the rest "happened" to come to Christ by the work of these Christians and the Holy Spirit? Or were ALL the people who God knew would become Christian through His omniscience thus "predestined" by Christ?

In 1 Corinthians 13, what is meant by the phrase "..thinketh no evil,.." and how does that relate to Biblical charity?

Will God, through the Holy Spirit, finally give up on calling someone for salvation, if the person continues to reject the call?

Can a Christian be turned over to a reprobate mind? I just wonder because there are some people who seem to fall away from God and never desire to go back.

I believe in eternal security but someone recently showed me a couple of passages from 2 Peter that were difficult.

In Hebrews 9:26 Paul (or whoever may be writing) said that Christ had appeared "once "IN THE END OF THE WORLD" to atone for sins. How can it be the "end of the world" when it happened almost 2,000 years ago (considering that this world is only 6,000 plus years old)?

In the Bible, does the '"day of the Lord" refer to the end times starting after the rapture? These verses suggest that the Day of the Lord starts with different periods in respect to the rapture. Joel 3:31, 1 Thessalonians 5:1-5 and Isaiah 13:9. Can you please explain these verses to me as to when exactly the day of the Lord begins?

When did the grace age begin and will it last 2,000 years?