When your hearts intentions are to build that person up. Not for proud reasons, like your wrong & I'm right, HA, let me show you in the bible. Do it with humility & grace, giving thanks to our Father, that he gave you this opportunity to enlighten.
When what they believe goes against the main beliefs of Christianity; Jesus died for our sins, rose again on the third day and that we must believe and accept his forgiveness to be able to go to heaven.
When the Spirit moves you. . I don't understand why this question is directed at one Christian to another. It makes me a little concerned because who will determine which Christian is really correct after all?
16 All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, 17 that the man of God may be complete, thoroughly equipped for every good work.
2 Timothy 2:23-26
23 But avoid foolish and ignorant disputes, knowing that they generate strife. 24 And a servant of the Lord must not quarrel but be gentle to all, able to teach, patient, 25 in humility correcting those who are in opposition, if God perhaps will grant them repentance, so that they may know the truth, 26 and that they may come to their senses and escape the snare of the devil, having been taken captive by him to do his will.