The seriousness of sin

As forgiven people, Christians can easily become complacent about returning to sinful ways. Though no true Christian would ever consciously think, 'God will forgive me' as they committed sin, yet there is undoubtedly a subconscious complacency that downplays the seriousness of sin and presumes on God's forgiveness.
Paul will have none of this. How can a child of God ever betray his Father like this? How could someone who fully appreciates the cost of his forgiveness ever presume?

Therefore be imitators of God, as beloved children. And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God.    Eph 5:1

So we must not sink into the dark and filthy mire of wickedness from which we were rescued:

But sexual immorality and all impurity or covetousness must not even be named among you, as is proper among saints. Let there be no filthiness nor foolish talk nor crude joking, which are out of place, but instead let there be thanksgiving. For you may be sure of this, that everyone who is sexually immoral or impure, or who is covetous (that is, an idolater), has no inheritance in the kingdom of Christ and God.     Eph 5:3-5

But some will say, 'Surely God will not be so harsh. Surely he will still forgive.' Paul will not have such presumption:

Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience.    Eph 5:6

Notice Paul doesn't say, 'The believer who gets involved in wickedness cannot be forgiven.' His comment is addressed to 'the sons of disobedience' - to unbelievers - but this just emphasises how unthinkable it should be for a true believer to return to the trough of disobedience. And one way to avoid this is to avoid being too involved with unbelievers. Yes, of course we need to be 'in the world' so that we can proclaim Christ to the world, but we mustn't get too involved because we're citizens of a different world:

Therefore do not become partners with them; for at one time you were darkness, but now you are light in the Lord. Walk as children of light (for the fruit of light is found in all that is good and right and true), and try to discern what is pleasing to the Lord. Take no part in the unfruitful works of darkness, but instead expose them. For it is shameful even to speak of the things that they do in secret. But when anything is exposed by the light, it becomes visible, for anything that becomes visible is light. Therefore it says,

“Awake, O sleeper,

and arise from the dead,

and Christ will shine on you.”

The non-Christian lives in a world of darkness and death. The Christian lives in a world of light and resurrection life.