Human beings come into the world imperfect and therefore inclined to error; sin puts a barrier against God, because God is perfect and does not coexist with error. But God is merciful and does not want to lose us, that is why the Lord provides us with forgiveness.
But the forgiveness that the Lord grants us implies a reciprocity, mortal sins cannot be forgiven because they are those that definitively break our relationship with God.
A mortal sin was the sin of the King of Babylon who mocked God with a sacrilege: "King Belshazzar gave a great banquet for a thousand of his nobles and drank wine with them. While Belshazzar was drinking his wine, he gave orders to bring in the gold and silver goblets that Nebuchadnezzar his father had taken from the temple in Jerusalem, so that the king and his nobles, his wives and his concubines might drink from them. So they brought in the gold tablets that had been taken from the temple of God in Jerusalem, and the king and his nobles, his wives and his concubines drank from them" Daniel 5:1-3.
A venial sin that does not break the relationship with God was the sin that David committed to marry Bathsheba, who was previously married to Uriah (2 Samuel 11).
When God forgives us it is with a precise purpose, that with forgiveness we can live in the fear of God, which is one of the most important gifts in the Bible, as explained in the Book of Psalms:
"If you, LORD, kept a record of sins,
Lord, who could stand?
But with you there is forgiveness,
so that we can, with reverence, serve you" Psalm 130:3-4
Serving God with reverence, as the Psalm explains, means fearing the Lord. The spiritual gift of fear of God is constancy, firmness, devotion, equanimity, and stability. This spiritual gift is what sustains our bond with God, and is the starting point of all spirituality in man, this is the gift through which we can receive the other spiritual gifts, like faith or hope, this spiritual gift is "the good land". The opposite of the fear of God is the hardness of the heart.