OUTLINE:

Unbelievers
The basic reason why someone rejects the Gospel is because its foundational premise is that they realize that a person NEEDS a Savior! If a person never rightly reconciles the chasm between themselves and the sovereign God of the Universe, they will miss this point altogether. There's a very real impetus for salvation.

An Impossible Chasm to Cross
Some people think the chasm between them and God is like jumping over a puddle, but instead it's like jumping over the entire Grand Canyon.

John the Baptist's Ministry of Repentance
The arrogant hearts of men need to be readied before they will turn to the Lord. If one doesn't admit their sinfulness and repent, they will reject Him, thinking they have no real need! Mat 3:1-12

Evil Has a Champion
Satan's ploy is to get people believing they are righteous without God's help...or at least "enough" to gain entrance into Heaven (even the end goal is perverted). He propagates lies stating that if you're self-righteous enough, you certainly don't need Christ-righteousness.

The Gospel Hope
The Gospel is EVERYTHING to a believer. Any hope of deliverance at or after being saved is a function of this one reality.

"if you hold fast the word"
Paul was concerned about the quality of what people believed in, and whether or not said belief resulted in true faith (2Co 13:5; 1Jn 2:19). "For by grace you have been saved through faith" (Eph 2:8a). The Gospel of salvation was the centerpiece of his ministry, always.

Paul's Calls to Examine
Paul was never timid about having individuals examine their own faith to see if they were truly disciples... and throughout scripture, you see him inserting this kind of testing into the hearts of those he was writing to.

2Co 13:5 [Amplified]
Test and evaluate yourselves to see whether you are in the faith and living your lives as [committed] believers. Examine yourselves [not me]! Or do you not recognize this about yourselves [by an ongoing experience] that Jesus Christ is in you-unless indeed you fail the test and are rejected as counterfeit?

Why Does the True Believer Persevere?
One reason only; because God has literally changed them, made them new creatures in Christ, and then gave them His own Holy Spirit to empower them!

1Jn 2:19
They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.

"first importance"
from protos - implies a superlative; means foremost (in time, place, order of importance); in context, places the Gospel as the centerpiece of Paul's ministry

"and so you believed"
This is the very reason, the intended result, of Paul's preaching. This wasn't some diatribe against Corinthian immaturity, it was a clear indication of the goal of Paul's ministry - "to seek and to save" (ala Lk 19:10).

Jesus NEVER had a problem challenging a person's salvation.

Mt 12:32
"Whoever speaks a word against the Son of Man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come."

1Th 5:21-22 [Amplified]
But test and prove all things [until you can recognize] what is good; [to that] hold fast. Abstain from evil [shrink from it and keep aloof from it] in whatever form or whatever kind it may be.

"examine everything carefully"
Never take anyone's word at face value. Examine the things you hear from fellow man carefully, lest you be lazy and accept false doctrines as truth. "A man who stands for nothing falls for anything."

"By Grace, They Were Prepared"
Not only did their natural abilities have nothing to do with Jesus choosing them; but they had nothing to do with their preparation for ministry. In fact, their natural "abilities" handicapped them.

Sending the Apostles Out
• Jesus called them
• Jesus trained them (academically & OJT)
• Jesus sent them out

"By Grace, They Were Prepared"
We need to understand when/how the apostles' natural abilities were insufficient for the commission they were eventually given. It wasn't enough that Jesus simply called 12 "unexceptional" men. He made a point of training them up before sending them out.