Romans 15:14
And concerning you, my brethren, I myself also am convinced that you yourselves are full of goodness, filled with all knowledge, and able also to admonish one another. --
Paul is writing to Gentile Christians in Rome before he has been able to go to Rome to meet them. But from what he has heard about them, he concludes that God has given them His goodness, and that they have studied or been taught the knowledge of God's Truth; this is what qualifies them to "admonish one another."
So when we perceive that another brother or sister has slipped into some wrong or deceptive or harmful act or thought process, then we must examine ourselves first to make sure we have God's goodness in us and have accurate knowledge, then we will be qualified to admonish them, but only in love (see I Corinthians 13).
Even so, come, Lord Jesus!
