42 Lk 22.31-32 Sin, plus repentance, does not constitute failed faith
<p><b>Luke 22:31-32 (ESV) </b>
<sup>31 </sup> “Simon, Simon, behold, Satan demanded to have you, that he might sift you like wheat,
<sup>32 </sup> but I have prayed for you that your faith may not fail. And when you have turned again, strengthen your brothers.”</p>
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Note that Peter would, as Jesus suggested by "when you have turned again", sin by denying Jesus but would then repent of the sin and "turn again". Since none of Jesus' prayers were ever denied by the Father, His prayer that Peter's faith would not fail was granted -- so though Peter sinned, his faith did not fail.
<b>Conclusion</b>: sin, with subsequent confession and repentance, does not constitute a failure of one's faith.
In contrast, sin, with no repentance, such as the person who professes to believe in Christ but lives a life of unrepentant sin, indicated by the use of the Greek present tense in "continue to sin" in 1 Jn 3.4-10, indicates someone who actually does not have the faith of a saved person, a true "believer" in Christ.
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