That is not to say, however, that sin committed after being born again cannot have any effects on the believer. It may not separate him or her from the love of God in Christ Jesus, but it can adversely effect what we do experience in God. Let us look at a couple of possibilities (not to say there are not others).
Sin Can Hinder Our Prayers
If anyone cherishes (i.e, hides like a treasure) some sin in his or her heart, the Lord is not obligated to look past that and hear them as if they were dealing with God sincerely. Peter applies this principle specifically to abusive Christian husbands (never mind the contradiction in terms), so it is not merely an OT construct. There is, of course, a difference between sinning and cherishing sin in the heart, but the latter at least seems to raise a question in the mind of God as to whether or not that one truly has faith in Christ.
For those that acknowledge sin as sin (i.e, they say the same thing as God does about it), they have no "sound barrier" with God. They confess it, he is faithful and just and forgives their sin and cleanses them from all unrighteousness. They stand before God in Jesus’ stead (name), heard, and their prayers answered.
Sin Can Torpedo Our Faith
By not maintaining a good conscience, or not doing what you know God would have you do, one can undermine how his or her heart perceives God. It is not something that happens in an instant. Over time, if one continues to act by the principle that sin doesn't matter, or by the assumption that God won't mind, eventually that one will come to the conclusion that God doesn't matter. Even if his words never say it, his faith as reflected in his actions will betray his absolute lack of trust in God. God can never be fooled, and faith can be shipwrecked.
Nonetheless, notwithstanding these considerations, the principle is clear and founded upon an unmutable fact of history, my sin has been put aside. Jesus became my sin and suffered my punishment; therefore, I am justified in Christ. I can fellowship with God, just as if I'd never sinned; talk with God, just as if I'd never sinned; experience peace with God, just as if I'd never sinned; and countenance no condemnation, just as if I'd never sinned! Sin may have its effects on the Christian, but thankfully Christ has a better effect on the Christian's sin.