St. Augustine and Pelagius debated this topic in the 4th century. To answer that, we must consider the intent and design of the atonement. The Arminianist position holds that Christ's saving work makes possible salvation for all men on the condition of faith, but does not guarantee or secure the salvation of any person. The atonement is limited in effectiveness. It saves no one, but makes it possible for all people.
Salvation is conditioned on a person's faith.
The expiation and propitiation of Christ are conditioned on man. God can be frustrated when people reject Him, and He hopes some will be saved. The Calvinist position holds that Christ's saving work actually secures the salvation of certain people and not others. It saves only certain people. God is not frustrated, and He knows with certainty all He secured salvation for through Christ will be saved.