Based on your statement......the sacrifice is perfect and therefore applies to everyone..........therefore everyone will be saved regardless of their actions.
Not at all...that is the point at which "free will" comes into play. Obviously Adam and Eve had free will, but they choose the wrong path. As a result, we are all born into sin.
Presbyterians believe in what is commonly called the 5 Points of Calvinism (In fact, there are many Reformed Baptist churches around that adhere to the Calvinistic doctrine and many Baptist pastors that do also, although they dare not tell their congregation

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The acronym is T.U.L.I.P
T is for total depravity. We are all sinners in need of God's Grace and sin fills our hearts to the extent that we cannot make accurate decisions without God.
U is for unconditional election. This is very controversial to some. It basically means that before man, God choose his "elect" who would be saved. Basically, if God is all knowing, he will know who will choose Him.
L is for limited atonement. Atonement was intended to render complete satisfaction for those and only those whom the Father had chosen before the foundation of the world. Calvinists do not believe that the atonement is limited in its value or power (if the Father had willed it, all the people of all generations could be saved), but rather they believe that the atonement is limited in that it is designed for some and not all. (thanks be to wikipedia for the wordy definition). Once again, controversial to some. It makes sense if you think of it like this...if Christ died for everyone, everyone would go to heaven. It makes no sense that anyone would burn in hell in punishment for sin that had already been paid in full. I know, hard to accept.
I is for irresitible grace. God touches the heart of the person to be saved. If God decides for you to be saved, you most certainly will, as a result you decide to follow Christ. I bet everyone who is saved that is reading this didn't just up and decide to be saved, it came from the word of a pastor, parent, friend, etc...Presbyterians believe this is God's prompting.
P is for Preservation of the Saints. Once saved always saved...if God chooses you, you will not get away.
Presbyterians believe that we have free will, but due to our sinful nature (as Cam stated, far worse than most realize) we will choose the wrong thing every time. God has to change our heart or prompt us in the right direction.
Note: I did not grow up Presbyterian and I realize that this sounds very weird to many people. I came to this understanding after much research. Plain language in the Bible supports all of this above. By the way, this does not mean that God fearing deacon will go to hell and Tommy Lee to heaven.
SORRY ABOUT THE UNFORSEEN DOUBLE POST