In discipleship discussions, we often quote 2 Timothy 2:2 as a pillar text for understanding discipleship that multiplies disciples. The idea, of course, is that Paul invested in training Timothy and was encouraging him to invest in other people who would invest in others.
Notice the adjective that Paul uses to describe the people in whom Timothy was to invest his time. Depending on your translation, the word is generally reliable (NIV, NLT), or faithful (ESV, NASB, KJV). It is someone in whom you trust.