I could believe it, actually. Augustine of Hippo relates a plausible example of this, even in infancy.
“I have myself”, he says, “observed a baby to be jealous, though it could not speak; it was livid [its face would be pale and envious, says another translation] as it watched another infant at the breast.”
And of course, I don’t need to tell you Augustine committed far worse things later on in life.
I could believe even infants could, objectively, commit sin.
Even if they could not, that’s not the question. The question is: do we all have original sin? Original sin is a different thing entirely. Per #402 of the Catechism, and Romans 5: “sin came into the world through one man and death through sin, and so death spread to all men because all men sinned.” We are not guilty of original sin. We suffer the consequences of it - death, concupiscence ( a tendency to sin, generally), and a lack of saving grace.
Adam and Eve did not know concupiscence or death, and they were full of grace. By their first sin - whatever that was, it was disobedience and distrust of God - they introduced human beings and the world to distrust, disobedience, and death. Like any idea, like any product, like any addiction, it never can go away entirely.
This is evident in many things. If it can be thought of, it can be done, and anyone can do it. I am capable of a number of things I would hate - murder, drug addiction, neglect, abuse, rape, cheating, lying, theft, adultery, lust, becoming a Baptist. There is not one sin of which I am not capable, and I must be on guard against them all.
If this seems extremist, think of the saints. Some of them were full of lust, neglect, selfishness, before they met Jesus and His Body the Church. I consider saints like St. Mary of Egypt and St. Augustine of Hippo my patrons, because they knew how to hate God. And they know how to love God despite that.
