To sin against the love of God

The contest is for our lives and souls. Not to be daily employing the Spirit and new nature for the mortifying of sin is to neglect that excellent succor [assistance] which God has given us against our greatest enemy. If we neglect to make use of what we have received, God may justly hold his hand from giving us more. His graces, as well as his gifts, are bestowed on us to use, exercise and trade with. Not to be daily mortifying sin is to sin against the goodness, kindness, wisdom, grace, and love of God, who has furnished us with a principle of doing it. (Overcoming Sin and Temptation p54)

To neglect of the means of grace is to despise what is given, and the One who gives it, and is therefore to sin against the very love and mercy of God.