"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." (Genesis 4: 8)
Men and women who look for reasons to fault and diminish the Bible will point to passages like the one above.
If Abel's sacrifice was accepted, and God had respect for it, then how come God did not protect Abel from Cain's wrath?
Let us consider what led to Cain and Abel meeting, from a different translation:
"And Cain saith unto Abel his brother, 'Let us go into the field;' and it cometh to pass in their being in the field, that Cain riseth up against Abel his brother, and slayeth him." (YLT)
Cain invited Abel to the field, the same cursed ground from which Cain brought fruits, picture of his self-effort, acting as though his works could cover up sin.
Why would Abel want to meet with Cain on cursed ground? Here we have a picture of righteous people going back to their efforts.
Paul expressly warned the Galatians about going from grace back to law:
"O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you?" (Galatians 3: 1)
"Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace." (Galatians 5: 4)
When you know that you have been made the righteousness of God in Christ (2 Corinthins 5: 21), do not go back to "tilling the soil", trying to perfect yourself. Instead, grow in grace and knowledge of the Lord Jesus (2 Peter 3: 18)