The reason is simple. Christians are saved by faith in the work of Jesus on the cross. But faith in itself is not enough. Faith is not a substance you can put in a jar. It is belief in something.
Faith is only as good as who it is placed in.
If you put your faith in a false God, you are lost because a false god cannot save anyone. F aith is not what saves, but faith in the true God is what saves. Heresy has the ability to damn because they have the ability to confuse the gospel sufficiently to make it powerless. For this reason, I suspect that to many ancient Christians, heresy became one of the most serious of offenses. It becomes necessary to define those doctrines which separate Christian from non-Christian.