Genesis 4:7

“If thou doest well, shalt thou not be accepted? etc.] That is, either if thou doest thy works well in general, doest good works in a right way and manner, according to life will of God, and directed to his glory, from right principles, and with right views: so all the Targums, “if thou doest thy works well;” for it is not merely doing a good work, but doing the good work well, which is acceptable to God; hence that saying, “that not nouns but adverbs make good works:” or particularly it may respect sacrifice; if thou doest thine offering well, or rightly offereth, as the Septuagint; or offers not only what is materially good and proper to be offered, but in a right way, in obedience to the…”

“If thou doest well, shalt thou not be accepted?--A better rendering is, "Shalt thou not have the excellency"? which is the true sense of the words referring to the high privileges and authority belonging to the first-born in patriarchal times. sin lieth at the door--sin, that is, a sin offering--a common meaning of the word in Scripture (as in Hos 4:8; Co2 5:21; Heb 9:28). The purport of the divine rebuke to Cain was this, "Why art thou angry, as if unjustly treated? If thou doest well (that is, wert innocent and sinless) a thank offering would have been accepted as a token of thy dependence as a creature. But as thou doest not well (that is, art a sinner), a sin offering is necessary, by…”

“- Section IV - The Family of Adam - Cain and Abel 1. קין qayı̂n, Qain (Cain), “spear-shaft,” and קנה qānah, “set up, establish, gain, buy,” contain the biliteral root קן qan, “set up, erect, gain.” The relations of root words are not confined to the narrow rules of our common etymology, but really extend to such instinctive usages as the unlettered speaker will invent or employ. A full examination of the Hebrew tongue leads to the conclusion that a biliteral root lies at the base of many of those triliterals that consist of two firm consonants and a third weaker one varying in itself and its position. Thus, יטב yāṭab and טיב ṭôb. So קין qayı̂n and קנה qānah grow from one root. 2. הבל…”

“If thou dost well, shalt thou not be accepted? - Either, 1. If thou hadst done well, as thy brother did, thou shouldest have been accepted as he was. God is no respecter of persons; so that if we come short of acceptance with him, the fault is wholly our own. This will justify God in the destruction of sinners, and will aggravate their ruin. There is not a damned sinner in hell, but if he had done well, as he might have done, had been a glorified saint in heaven. Every mouth will shortly be stopt with this. Or, 2. If now thou do well: if thou repent of thy sin, reform thy heart and life, and bring thy sacrifice in a better manner; thou shalt yet be accepted. See how early the gospel was…”

“4:7 Sin is crouching at the door . . . you must subdue it: Sin is pictured as a vicious animal lying in wait to pounce on Cain (cp. study note on 3:16). Either sin will dominate Cain, or Cain will resist the temptation to sin. There is no neutral ground in that conflict.”

“If thou doest well: Gen 19:21; Sa2 24:23; Kg2 8:28; Job 42:8; Pro 18:5; Ecc 8:12, Ecc 8:13; Isa 3:10, Isa 3:11; Jer 6:20; Mal 1:8, Mal 1:10, Mal 1:13; Act 10:35; Rom 2:7-10, Rom 12:1; Rom 14:18, Rom 15:16; Eph 1:6; Ti1 5:4; Pe1 2:5 be accepted: or, have the excellency, Job 29:4; Pro 21:27; Heb 11:4 sin: Gen 4:8-13; Rom 7:8, Rom 7:9; Jam 1:15 unto thee: or, subject unto thee, Gen 3:16 *marg. Genesis 4:8”