Correction, the natural man cannot understand spiritual truth. 1 Cor 2:14
But the Spiritual man/regenerated man can 1 Cor 2:12-13
12
Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
13
Which things also we speak[in the Gospel], not in the words which man's wisdom teacheth,
but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
See God regenerates a man with the Spirit in order that they man KNOW Spiritually the things of the Gospel which announces the things freely given to them
The words freely given in the original charizomai:
Here its in the passive voice:
to do something pleasant or agreeable (to one), to do a favour to, gratify
- to show one's self gracious, kind, benevolent
- to grant forgiveness, to pardon
- to give graciously, give freely, bestow
- to forgive
One of those things freely given is forgiveness of sins, pardon, justification
Eph 4:32
And be ye kind one to another, tenderhearted, forgiving one another, even
as God for Christ's sake hath forgiven you.
Its used when Paul writes its been given to believe on Christ Phil 1:29
For unto you it is given in the behalf of Christ, not only to believe on him, but also to suffer for his sake;
So this scripture tells me that believing in Christ is caused by Grace
charizomai
JFB says of Phil 1:29
it is given—Greek, "it has been granted as a favor," or "gift of grace." Faith is the gift of God (Eph 2:8), not wrought in the soul by the will of man, but by the Holy Ghost (Joh 1:12, 13).
You failed to explain how it is these unregenerates were able to believe
John 12:40 (KJV 1900) — 40 He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.
God blinded men to prevent their belief. Why would God blind someone who had no ability to see?
Why prevent from believing those who had no capacity for belief
Luke notes had men not grown hardened they could have believed
Acts 28:27 (KJV 1900) — 27 For the heart of this people is waxed gross, and their ears are dull of hearing, and their eyes have they closed; lest they should see with their eyes, and hear with their ears, and understand with their heart, and should be converted, and I should heal them.
Jesus clearly states he hides truth for some as otherwise they would repent and be forgiven
Mark 4:10–12 (ESV) — 10 And when he was alone, those around him with the twelve asked him about the parables.
11 And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables,
12 so that “ ‘they may indeed see but not perceive, and may indeed hear but not understand, lest they should turn and be forgiven.’ ”
unregenerate men are shown to receive the word with joy and believe
Luke 8:13 (KJV 1900) — 13 They on the rock are they, which, when they hear, receive the word with joy; and these have no root, which for a while believe, and in time of temptation fall away.
Mat 11:21-24 Woe unto thee, Chorazin! woe unto thee, Bethsaida! for if the mighty works, which were done in you, had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes. 22 But I say unto you, It shall be more tolerable for Tyre and Sidon at the day of judgment, than for you. 23 And thou, Capernaum, which art exalted unto heaven, shalt be brought down to hell: for if the mighty works, which have been done in thee, had been done in Sodom, it would have remained until this day.
BTW to grant is to permit, allow , enable
It does not mean infuse or effectually cause
God permit allows enables faith by sending the gospel
God enabled the Philippians believe thusly
Acts 16:9–12 (KJV 1900) — 9 And a vision appeared to Paul in the night; There stood a man of Macedonia, and prayed him, saying, Come over into Macedonia, and help us. 10 And after he had seen the vision, immediately we endeavoured to go into Macedonia, assuredly gathering that the Lord had called us for to preach the gospel unto them. 11 Therefore loosing from Troas, we came with a straight course to Samothracia, and the next day to Neapolis; 12 And from thence
to Philippi, which is the chief city of that part of Macedonia, and a colony: and we were in that city abiding certain days.
sending them Paul by means of a vision to preach the gospel