“with the result that they indeed see but do not perceive.” This verse is quoted from Isaiah 6:9-10, and follows the Septuagint more closely than the Hebrew text. The “so that” is the translation of the Greek proposition hina which in this context shows the purpose of the parables. Jesus taught in parables with the result that the hearts of his listeners was revealed, and that is more clearly stated here in Mark than in any other of the Four Gospels.
Some New Testament Greek texts such as the Byzantine Text from which the KJV was translated, read “of sins” at the end of the verse, but scholars are now aware that this is an explanatory gloss that worked its way into the text.
[For this quotation from Isaiah and the purpose of parables, see commentary on Matthew 13:13.]
“otherwise they would turn to God.” The people who are sinning, who are so hardhearted that they will not even try to understand Christ’s parables, do not want to stop sinning, so they have no interest in knowing the will of God (cp. Job 21:14; 22:17; Isa. 30:11). If they did turn to God, He would heal them.
“and their sin would be forgiven.” The more literal translation is, “and it be forgiven them,” but the verb “be forgiven” is singular in the text, so it does not refer to the people but to the sin of the people.