The key passage about dealing with heretics and heterodox though is, imo, this one:
And he sent messengers before his face. And having gone they entered into a village of the Samaritans that they might make ready for him.
And they did not receive him, because his face was {turned as} going to Jerusalem.
And his disciples James and John seeing {it} said, Lord, wilt thou that we speak {that} fire come down from heaven and consume them, as also Elias did?
But turning he rebuked them {and said, Ye know not of what spirit ye are}.
And they went to another village.
Luke 9:52-56
We know that Samaritans were used as an example of honest faith. We know that Jesus talked to them always giving a straight witness calling them out of their mistakes. And here we see not apostolic successors, but the Apostles themselves being overzealous - they would be called fanatics - because they wanted to see the samaritans destroyed. Jesus properly tells them that such extremes are demonic in nature, but He does not enter the Samaritan village with the Apostles.
Jesus teaching about the heterodox is very clear: because they are serious about their faith and many times even more faithful than those who are doctrinally correct, they are not to be treated as enemies of God. You can and should meet with them to deal about everyday things of life like doing business, but when meeting with them about theological issues, the witness has to be firm and outright: leave your mistakes and come to the fullness of truth.
The same rebuke to "fire from the sky" kind of zeal was expressed in another passage, the Parable of Tares, where Jesus affirm very clearly that any attempt at fixing the problem of coexistance of virtues and sins, correct and wrong doctrines, orthodoxy and heresy is doomed to failure because they will exist to the end of time when God Himself will set each apart (and not unite them).
The problem with unionist ecumenism is that because of praying for a fire from the sky to destroy the heretics, they argue that tare and wheat, samaritans and jews are deep down the same thing. But like the zealots, they are trying to fix a problem that Jesus said is not to be fixed and will not be fixed before the end of the world.