Where does the sin in infants come from?
In their Representative head ! When Adam sinned, all whom He represented sin also according to God's word. Rom 5:12,19
12Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men,
for that all have sinned:
19For
as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
The word sinner here is the greek word hamartōlos:
devoted to sin, a sinner
a) not free from sin
b) pre-eminently sinful, especially wicked
1) all wicked men
2) specifically of men stained with certain definite vices or crimes
a) tax collectors, heathen
This word is from the root word hamartanō:
to be without a share in
2) to miss the mark
3) to err, be mistaken
4) to miss or wander from the path of uprightness and honour, to do or go wrong
5) to wander from the law of God,
violate God's law, sin
And since it means violate God's Law, it also can be rendered Adam's Transgression Rom 5:14
14Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of
Adam's transgression, who is the figure of him that was to come.
Transgression here is the word parabasis:
a going over
2) metaph. a disregarding,
violating
So all who were in Adam, and God constituted him their head, when he sinned or transgressed, they did also, before they were born. Now when they were born they were born as infants, with their transgression already having been committed in Adam their head, this is according to scripture truth whether you, me or anyone else believes it or not !