My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous (1 John 2:1)
If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. (1 John 5:16)
Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. (1 John 3:9)
Sometimes, one will hear that the First Epistle of John is internally contradictory on the question of whether Christians can sin, as per 1 John 2:1 and 5:16 or cannot sin, per 1 John 3:9. However, such reflects a rather shallow approach to these biblical texts and the theology thereof; as one Arminian scholar noted:
[W]hat are we to say about 2:1: “If anyone does sin, we have an advocate with the Father, Jesus Christ the righteous” and 5:16, “If anyone sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal”? Of course, one’s theological biases could dictate an argument that relegates these references to those who have not yet come to faith in Christ. The clear sense of both passages, however, seems to point toward the sins of believers. How is it, then, that 3:9 says that the believer cannot sin while 2:1 and 5:16 speak of that very possibility? Those who attribute the apparent contradiction to a discrepancy internal to the thought off the writer himself as well as those who attribute it to careless redactional work have solved it, each in a different way.
However, what appears to be contradictory is explainable within the theological context of 1 John itself, if, indeed, one assumes the distinction that he seems to be making between one’s ego status before God, on the one hand, and the particularities of one’s lived-out existence, on the other. The particularities of one’s lived-out existence can be phenomenologically viewed as sin, both by God and by us. The writer of this epistle is congnizant of the not-yetness will be eradicated only in glorification. As long as the believer lives in transit from the time of divine birth to the time of glorification, his or her lived-out existence will have particularities about it that fall short of the glory of God. What is being said in this connection perhaps can be understood best by observing that a nonbeliever can likewise live out the particularities of life in such a way that phenomenologically the person looks like a believer. Who the person is at the core of his or her ego is one thing; what he or she appears to be phenomenologically is another . . . We are led to the conclusion, therefore, that even for believers, there are discrepancies between the phenomenological particularities of our existence, on the one hand, and who we are at the core of our ego, on the other. Believers have an “advocate with the Father” (2:1) to whom they can confess such wins, which in 5:17 are called hamartia ou pros thanaton, “sin not unto death.” It is assumed that the believer will not make no peace with such sin but will avail herself or himself of the advocacy of Jesus Christ whenever the Spirit and the prayerful community of faith bring it to the attention of his or her heart. The believer does not make peace with harmatia ou pros thanaton, but rather “purifies himself” (3:3). (Gilbert W. Stafford, “Salvation in the General Epistles” in An Inquiry into Salvation: From a Biblical Theological Perspective, eds. John E. Hartley and R. Larry Shelton [Anderson, Ind.: Warner Press, Inc., 1981], 195-224, here, pp.216-18)