It must be kept in mind that Adam and Eve were free moral agents.
That while they were sinless beings, it was yet possible for them
to sin, just as it was possible for them not to sin. A careful
reading of the narrative leads to the following remarks:
The sin of our first parents was purely volitional; it was an act
of their own determination. Their sin was, like all other sin, a
voluntary act of the will.
It came from an outside source, that is to say, it was instigated
from without. There was no sin in the nature of the first human pair.
Consequently there must have been an ungodly principle already in
the world. Probably the fall of Satan and the evil angels had taken
place already.
The essence of the first sin lay in the denial of the divine will;
an elevation of the will of man over the will of God.
It was a deliberate transgressing of a divinely marked boundary;
an overstepping of the divine limits.
In its last analysis, the first sin was, what each and every sin
committed since has been, a positive disbelief in the word of the
living God. A belief of Satan rather than a belief in God.
It is helpful to note that the same lines of temptation that were
presented to our first parents, were presented to Christ in the
wilderness (Matt. 4:1-11), and to men ever since then (1 John
2:15-17). Satan's program is short and shallow after all.