Partaker - sharing. From the very beginning there was a sharing of God with people whether saved or not -So they're not as described in the text, i.e., brethren, partakers of the Holy Spirit, tasted the good word of God, received the knowledge of the truth, sanctified by the blood of the covenant, the LORD's people?
God shared with Cain what His expectations were after Cain expressed his dismay of not being accepted.
Genesis 4
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
Cain heard the good word of God (which He shared with Cain) and the good things to come as God responded to his dismay.
Genesis 4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
Cain rejected and rebelled from the word of God and murdered his brother Abel.
Genesis 4:8 And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.
Cain prayed and complained to God after he murdered his brother and God respected his prayer protecting him from the rest of mankind.
Genesis 4:13 And Cain said unto the LORD, My punishment is greater than I can bear.
14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me.
15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
Just like Cain, every human being coming into the world has light from God.
John 1
6 There was a man sent from God, whose name was John.
7 The same came for a witness, to bear witness of the Light, that all men through him might believe.
8 He was not that Light, but was sent to bear witness of that Light.
9 That was the true Light, which lighteth every man that cometh into the world.
Even unbelievers have a measure of light from God:
John 16
7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
8 And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
9 Of sin, because they believe not on me;
Some theologians believe that after a season of rebellion the Spirit of God will leave off convicting a man and depart from him never to return but I believe it's the opposite:
Genesis 4:16 And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.
Felix and Agrippa (IMO) departed from the convicting/reproving work of the Spirit because of Paul's preaching/teaching.
So it is with every one who hates God.
They know He exists and they know His expectations, but they will NOT repent.
Instead they blaspheme Him.
Revelation 16
9 And men were scorched with great heat, and blasphemed the name of God, which hath power over these plagues: and they repented not to give him glory.
10 And the fifth angel poured out his vial upon the seat of the beast; and his kingdom was full of darkness; and they gnawed their tongues for pain,
11 And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds.
HankD