Psalm 66:16-20 says:
Come and hear, all you who fear God,
and I will tell you what he has done for my soul.
I cried to him with my mouth,
and high praise was on my tongue.
If I had cherished iniquity in my heart,
the Lord would not have listened.
But truly God has listened;
he has attended to the voice of my prayer.Blessed be God, because he has not rejected my prayer
or removed his steadfast love from me!"
Here the psalmist asserts a hard truth: if he had "cherished iniquity", the Lord would not have listened. What does cherishing iniquity look like? Aren't we as humans (though we may be saved) sinful creatures, prone to iniquity?
There's a difference between stumbling to sin and loving sin. Yes, we are prone to sin due to the lingering sin nature that remains in us, but it is possible to sin and repent, sorrowful of spiting our Savior. Paul describes this state well:
For we know that the law is spiritual, but I am of the flesh, sold under sin. For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. Now if I do what I do not want, I agree with the law, that it is good. So now it is no longer I who do it, but sin that dwells within me. For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out. For I do not do the good I want, but the evil I do not want is what I keep on doing. Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me.
So I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members. Wretched man that I am! Who will deliver me from this body of death?
"Wretched main that I am," cries a desperate Paul, sorrowful and repentant of his own sinful nature, longing to live according to the perfect law of God. This is sin with real repentance.
It is also possible to be unwilling to repent of a sin, to love a sin so much that we shut out our conscience, God's word, and the counsel of other believers. In Proverbs we see this self-seclusion:
Whoever isolates himself seeks his own desire;
he breaks out against all sound judgment.
A fool takes no pleasure in understanding,
but only in expressing his opinion.
A fool (the sort who rebels against God and ultimately comes to ruin) isolates himself or herself, shutting out "all sound judgment" in order to seek his or her own desire. In so doing, such a person elevates his or her own desires and opinion to the highest position of wisdom, self-justifying and discarding submission to God's declared truth altogether. In short, such a person usurps God's roles as lawgiver and judge.
Because this kind of behavior supplants God, it is foolish to think that we could approach him in his favor.