I believe that the idea of original sin is Biblical.  Romans – the book of the Bible that deals fairly exclusively with how salvation is obtained – and also how sinful man actually is says this in chapter 5: 12-14 “Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned – for sin indeed was in the world before the law was given, but sin is not counted where there is not law.  Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.”      And later in that chapter, verse 19 “For as by the one man’s disobedience the many were made sinners, so by the one man’s obedience the man will be made righteous.” 

The psalmist David confesses in Ps 51 verse 5 “Behold, I was brought forth in iniquity, and in sin did my mother conceive me”.  

I have the Reformation Study Bible – ESV in front of me, and let me share a little bit of what RC Sproul writes regarding original sin:

“Original sin” meaning sin derived from our origin, is not a biblical phrase (it comes from Augustine), but it does bring into focus the reality of sin in our spiritual system.  Original sin does not mean that sin belongs to human nature as such; “God made man upright” (Eccl 7:29), Nor does it mean that the process of reproduction and birth are sinful; the uncleaness associated with sexuality in the Law (Lev 12, 15) was typical and ceremonial, not moral.  Rather, original sin means that sinfulness marks everyone from birth, in the form of a heart inclined toward sin, prior to any actual sins; this inner sinfulness is the root and source of all actual sins; it is transmitted to us from Adam, our first representative before God.  The doctrine of original sin makes the point that we are not sinners because we sin, but we sin because we are sinners, born with a nature enslaved to sin.

The phrase “total depravity” is commonly used to make explicit the implications of original sin.  It signifies a corruption of our moral and spiritual nature is total in principle although not in degree ………No part of  us is untouched by sin, and no action of ours is as good as it should be.  Consequently, nothing we do is ever meritorious in God’s eyes……………… Total depravity includes total inability, that is, being without power to believe in God or His Word (John 6:44, Rom 8 : 7,8) Paul calls this universal unresponsiveness a form of death; the fallen heart is “dead” (Eph 2:1.5; Col 2:13).”   (end quote)

Some of the comments here show a remarkable ignorance of what some protestants believe with regards to infant baptism.  There is more to it than just what the Roman Catholics believe about original sin and their (false) understanding of how baptism supposedly saves.   Protestants who hold to infant baptism, do not believe that baptism saves anyone.   It has to do with covenant belief related to the covenant that God established with Abraham and his descendants.  But that’s a whole other topic!!!  Do I believe then that with or without baptism an infant who dies is not saved?  No I believe God’s grace is more bountiful than we can imagine.    And more specifically, that children of believers who die before the age of accountability (which I think is different for every person) whether they are baptized or not, God will save because of the belief of their parents and because of the promises extended to the children of believing parents.     

PS-  I don’t believe that this is the place to argue infant baptism vs believers baptism, I just wanted to point out that there seems to be a misunderstanding about protestant infant baptism.