Gal 5:4 "Christ is become of no effect unto you, whosoever of you are justified (present tense) by the law; ye are fallen (aorist tense; indicative mood) from grace."
In writing this epistle "unto the churches of Galatia" Paul says the ones who were trying to justified by the law "are fallen from grace"
THe present tense of the verse shows that some had actually fallen from grace. The aorist tense show they were already fallen and the indicative mood is a statement of fact, that is, the falling had already, really occurred.
Logically, one cannot fall from God's grace unless he was first in God's grace.
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Heb 6:4,5 "For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come,"
The Hebrew writer is referring to those that are Christians.
Heb 6:6 "If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
The KJV, RSV along with some other versions added the first word "if" in an attempt to show the possibility of a Christian falling away. But the word "if" is not in the original texts.
The literal translation of the verse better reads "and having fallen away". The aorist tense shows the falling away has actually happened to some and not just a possibility.
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2 Pet 2:20-22 "For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire."
Peter refers to Christians that have escaped the pollution (defilement, influence of sin) of the world though the knowledge of Christ (Heb 10:26). The Christians can become "again entangled". Logically one must be out of the entanglement of the pollution of this world for him to become "AGAIN" entangled. One cannot always be entangled and become "AGAIN" entangled.
"and overcome, the latter end is worse with them than the beginning".
If they were lost before becoming a Christian and now lost on the latter end, why then is being lost in the end worse than being lost at the beginning? After all, lost is lost. The latter end is worse for in the begiinning before becoming a Christian they sinned in ignorance but after coming into the knowledge of Christ they obeyed the gospel and became a Christian. Yet being lost in the end is worse for they are lost having had the knowledge of Christ but simply now do not care they are lost and are unwilling to do anything about their lost conditions. At the beginning they were willing to do something about their lost state, but in the end they have no concern now for their lost state. As those in Heb 6:6, after becoming Christians but now fallen away in the end, they now have no concern about their salvation seeing they continue to put Christ to shame and continue to crucify Him.
"But it is happened unto them"
The verb 'happened" is perfect tense, indicative mood. Perfect tense shows it has happened, not possibly could happen but as a matter of fact (indicative mood) did happen where they actually, really are "again entangled" and have "turn from the holy commandment delivered unto them."
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Heb 3:12 "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God."
"take heed" a warning. No sense in warning about something that cannot, will not happen. The warning itself implies it can happen.
"brethren" Christians
"lest there be in any of you an evil heart of unbelief," Unbelief is from the heart.
"departing from the living God" one cannot depart from God if he were never with God.