Why are some truths in the Gospel hidden from certain people?

In Mark 4:12, Jesus states that truths are concealed from those outside the faith to illustrate the staggering reality of spiritual blindness. This purposeful concealment serves to underline the total reliance on God's grace for understanding and salvation. The blind-hearted reject the Gospel and, as such, are not given insight into its mysteries. This mirrors God’s sovereign decree as evidenced in Romans 11:7, where it is indicated that the elect will be called to God, while others will remain in darkness. The parables serve as tools that both reveal and conceal, thus demonstrating the divine justice and wisdom inherent in God's plan of salvation, highlighting the necessity of the Holy Spirit's illuminating work.