What follows is an excerpt from Thomas Watson's A Body of Divinity, (4:2:104-05), on the seriousness of the first sin by Adam. (Slightly edited for archaic terms)...
The aggravation of Adam’s sin. Wherein did it appear to be so great? It was but the seizing of a fruit. Was it such a great matter to pluck an apple? It was against an infinite God. It was malum complexum, a voluminous sin, there were many twisted together in it; as Cicero says of parricide, ‘He who is guilty of it, he commits many sins in one;’ so there were many sins in this one sin of Adam. It was a big-bellied sin, a chain with many links. Ten sins were in it.
(1) Unbelief. Our first parents did not believe what God had spoken was truth. God said that they shall die the death in the day they eat of that tree. They believed not that they should die; they could not be persuaded that such fair fruit had death at the door. Thus, by unbelief they made God a liar. Worse: they believed the devil rather than God.
(2) Ingratitude, which is the epitome of all sin. Adam’s sin was committed in the midst of Paradise. God had enriched him with variety of mercies; he had stamped his own image upon him; he had made him lord of the world; gave him of all the trees of the garden to eat (one only excepted), and now to take of that tree! This was high ingratitude; it was like the dye to the wool, which makes it crimson. When Adam’s eyes were opened, and he saw what he had done, well might he be ashamed, and hide himself. How could he who sinned in the midst of Paradise, look God in the face without blushing!
(3) Discontent. In Adam’s sin was discontent. Had he not been discontented, he would never have sought to have altered his condition. Adam, one would think, had enough, he differed but little from the angels, he had the robe of innocence to clothe him, and the glory of Paradise to crown him; yet he was not content, he would have more; he would be above the ordinary rank of creatures. How wide was Adam’s heart, that a whole world could not fill it!
(4) Pride, in that he would be like God. This worm, that was but newly crept out of the dust, now aspired after Deity. ‘Ye shall be as gods,’ said Satan, and Adam hoped to have been so indeed; he supposed the tree of knowledge would have anointed his eyes, and made him omniscient. But, by climbing too high, he got a fall.
(5) Disobedience. God said, ‘Thou shalt not eat of the tree;’ but he would eat of it, though it cost him his life. Disobedience is a sin against justice. It is right we should serve him from whom we have our subsistence. God gave Adam his allowance, therefore it was but right he should give God his allegiance. How could God endure to see his laws trampled on before his face? This made him place a flaming sword at the end of the garden.
(6) Sinful curiosity. He meddled with that which was out of his sphere, and did not belong to him. God smote the men of Bethshemesh for looking into the ark. (I Sam 6:19.) Adam would be prying into God’s secrets, and tasting what was forbidden.
(7) Lust. Though Adam had a choice of all the other trees, yet his palate grew wanton, and he must have this tree. Like Israel, God sent them manna, angels’ food, ay, but they had a hankering after quail. It was not enough that God supplied their wants, unless he should satisfy their lusts. Adam had not only for necessity, but for delight; yet his wanton palate lusted after forbidden fruit.
(8) Sacrilege. The tree of knowledge was none of Adam’s, yet he took of it, and did sacrilegiously rob God of his due. It was counted a great crime to rob the temple, and steal the silver vessels; so it was in Adam to steal fruit from that tree which God had peculiarly enclosed for himself. Sacrilege is double theft.
(9) Murder. Adam was a public person, and all his posterity were involved and wrapped up in him; and he, by sinning, at once destroyed all his posterity, if free grace did not interpose. If Abel’s blood cried so loud in God’s ears, ‘The voice of thy brother’s blood crieth unto me from the ground,’ (Gen 4:40); how loud did the blood of all Adam’s posterity cry against him for vengeance!
(10) Presumption. Adam presumed upon God’s mercy; he blessed himself, saying he should have peace; he thought, though he did transgress, he should not die; that God would sooner reverse his decree than punish him. This was great presumption. What a heinous sin was Adam’s breach of covenant!
One sin may have many sins in it. We are apt to have slight thoughts of sin, and say it is but a little one. How many sins were in Adam’s sin! Oh take heed of any sin! As in one volume there may be many works bound up, so there may be many sins in one sin.