Here is how Saint Porphyrios puts its,
In debates, if you say a few words about religion you will prevail. Let the person who has a different opinion give free reign to his thoughts and speak as much as he likes… Let him sense that he is addressing himself to a calm and uncontentious person. Influence him though your graciousness and prayer and then speak briefly. You achieve nothing if you speak heatedly and tell him, for example, ‘What you are saying is untrue, a downright lie!’ What will you achieve? Be ‘as sheep among wolves’ (Matt 10:16). What should you do? Show indifference outwardly, but be praying inwardly. Be prepared, know what you are talking about and speak boldly and to the point, but with saintliness, meekness and prayer. But in order to be able to do this you must become a saint. This last thought is an important one:
“To be able to do this you must become a saint.” Ask yourself, "How do I become a saint?"
Reference: Wounded by Love, p 188