The five points of Calvinism represent the major tenets of Calvinistic thinking. They can be easily remembered by memorizing the acrostic TULIP: Total Depravity, Unconditional Election, Limited Atonement, Irresistible Grace, Perseverance of the Saints.
Many Christians take issue with the idea of a "limited atonement." Why would anyone say that the work of Christ is limited? This confusion may stem from the meaning of the term itself.
As Greg Koukl of Stand to Reason points out, the idea of a limited atonement should not be an issue. The atonement has to be limited in some sense, otherwise universalism (the idea that everyone will be saved) is true. How is that? Because Jesus accomplished something on the cross. He paid the penalty for sin. But if Jesus paid the penalty for sin, why isn't everyone saved? Why isn't universalism true?
Well for one, the Bible clearly does not teach universalism. Not everyone is going to heaven. So the question is this: How is the atonement limited?
A Calvinist would say that the atonement is limited in its scope. In other words, Christ did not die for every man, woman, and child. He gave His life up for the Church. He secured their redemption at the cross. This is why Christ could say, "I lay down my life for the sheep" (John 10:15). This view could more accurately be labeled "particular redemption" or "definite atonement." On this view, the death of Christ was sufficient for all but only efficient for some.
If you do not limit the atonement in its scope then it would seem you must limit it in its effect or nature. In other words, Christ did die for every man, woman, and child, but his atonement and sacrificial death did not actually secure anyone's redemption. It merely made salvation possible. Now it is up to the individual to place their trust in Christ in order to have the effects of the atonement applied to them.
Again, regardless of where you stand on this issue, one thing is for sure: the atonement must be limited in some sense. Greg Koukl explains in further detail:
Limited Atonement: Part 1 of 2
Limited Atonement: Part 2 of 2