These verses need to be correctly interpreted in order to avoid misleading people. Merely quoting them does no one any good.Those who are born of God cannot sin (1 John 3:9; 1 John 5:18).
1 John 3:9
πᾶς ὁ γεγεννημένος ἐκ τοῦ Θεοῦ ἁμαρτίαν οὐ ποιεῖ, ὅτι σπέρμα αὐτοῦ ἐν αὐτῷ μένει· καὶ οὐ δύναται ἁμαρτάνειν, ὅτι ἐκ τοῦ Θεοῦ γεγέννηται.
The interlinear translation of this verse is: Anyone -- having been born of God -- sin not practices, because seed of Him in him abides, and not he is able to continue sinning, because of God he has been born.
Expressing this idiomatically, it says: Anyone --having been born of God, DOES NOT PRACTICE SIN, because His [God's] seed abides in him. And he is not able TO CONTINUE SINNING, because he has been born of God.
This corresponds to the Bible truth that when a sinner is regenerated, he or she receives a new heart, a new spirit, a new nature, and "a new man". This new nature is consistent with God's nature, since it is indwelt by the Holy Spirit. So the "new man" does not practice sin nor does he continue sinning.
But this does not mean sinless perfection for the simple reason that the old Adamic nature -- "the flesh" -- has not been eradicated. And the flesh will cause Christians to stumble, sin, and fall, in spite of the new nature. Therefore Paul says that there is an inner conflict between the flesh and the Spirit. At the same time, Paul says that those who have crucified or mortified the flesh will walk in the Spirit.