1 John 1:9-2:2:
9 ἐὰν ὁμολογῶμεν τὰς ἁμαρτίας ἡμῶν, πιστός ἐστιν καὶ δίκαιος ἵνα ἀφῇ ἡμῖν τὰς ἁμαρτίας καὶ καθαρίσῃ ἡμᾶς ἀπὸ πάσης ἀδικίας. 10 ἐὰν εἴπωμεν ὅτι οὐχ ἡμαρτήκαμεν, ψεύστην ποιοῦμεν αὐτὸν καὶ ὁ λόγος αὐτοῦ οὐκ ἔστιν ἐν ἡμῖν. 1 Τεκνία μου, ταῦτα γράφω ὑμῖν ἵνα μὴ ἁμάρτητε. καὶ ἐάν τις ἁμάρτῃ, παράκλητον ἔχομεν πρὸς τὸν πατέρα, Ἰησοῦν Χριστὸν δίκαιον· 2 καὶ αὐτὸς ἱλασμός ἐστιν περὶ τῶν ἁμαρτιῶν ἡμῶν, οὐ περὶ τῶν ἡμετέρων δὲ μόνον ἀλλὰ καὶ περὶ ὅλου τοῦ κόσμου.
If any of us should confess our sins, then he who is faithful and just, will forgive our sins and cleanse us from all unrighteousness. If and of us should say, “We have not sinned,” then we make him a liar and his word is not in us. My children, I am writing this to you so that you will not sin, now if anybody should sin, we have an advocate with the Father, Jesus Christ, the just; and he is the atonement for our sins, but not merely for out sins, but for those of the whole world.
Notes: The teaching of 1 John poses a stumbling block for many Christians. It encourages Christians to live like Jesus Christ, “walk also as he walked.” There is no way around it, Christians live differently. To the person who wishes to live on the basis of wanton desire, 1 John gives no quarter. But to the legalist, 1 John also allows no comfort. John is quite clear, if somebody confesses their sin to God, with the intention of being cleansed and forgiven, God does just that. Any Christian can know that this is so for them, not because they do everything right, but because Jesus died for them. They can know Jesus died for them, because he died for the whole world. John wants Christians to live like Jesus, but Christians are sinners, so he reminds them that Jesus is their atonement. Christians, according to John, can have assurance of forgiveness by asking for it. They also can have assurance of salvation because they know they needed atonement in the first place! Elsewhere in the letter, which I'll get to, John explains that assurance also comes by believing that Jesus really lived in the flesh and by living a loving life.
I recommend that every Christian memorize this. It appears to be a cure for living like Jesus' teachings do not matter and for legalism.