First, let’s state what purgatory is not. Purgatory is not a second chance at salvation after death. If a person dies without God's grace, we know of no other chance or opportunity for that person to be saved. Instead, purgatory is for those who die in God's friendship, but have not yet achieved the holiness necessary to enter heaven. In other words, since nothing impure can enter heaven (Revelation 21:27), if we die without being perfectly pure, a purification must happen after death before we can enter heaven. So what things, one may ask, are purified in purgatory?
UNFORGIVEN VENIAL SINS ARE PURIFIED
The Catholic Faith teaches that when a person first comes to Christ, only their past and present sins are forgiven, but not their future sins (those sins that they are yet to commit). For example, in 1st John the apostle John is writing to fellow believers, and he says, "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness" (1 John 1:9). Now, if all of a believer's future sins are forgiven when they first come to Christ, then there shouldn't be any reason for a believer to have to confess his sins now, since they should already be forgiven. In other words, the only way that this verse would make sense, is if our future sins are not forgiven with when we first come to Christ.