But Acts 2:38 seems to state that being Baptized in the name of Christ cleanses you of sin.
Before we get really off course, let me say I agree with you here. But this is an example of the kinds of conflicts that arise when Christians presume to know who is going to . The church I grew up in believed Baptism was essential for entrance into Heaven. They believed members of churches who did not get baptized, or sprinkled water, were not true Christians. They used the letters of Paul as guidance for how worship was to be performed... No woman leaders, no instrumental music...
What does this have to do with the Scriptures you quoted? Well, you are defining "through Christ" by your set of rules. The Mormons believe in Christ. They ask his forgiveness for their sins just as you do. How can you know for sure, based on Scripture, that they are condemned?
Some Christians believe Catholics are damned. Some believe Methodists and Lutherans are damned... Some Church of Christ leaders believe half of the Churches of Christ in the country have veered away from Scripture to the point that THEY are damned.
It's a cluster. All because those Christians are so obsessed with deciding who's going to instead of ministering to people on the glory of Heaven. Do they condemn others because they are concerned for their souls? No. They condemn them in order to congratulate themselves for being God's Chosen. And I get the feeling that, if they had a choice, they would keep the list as exclusive as possible.