As to your opponent, I wish that before you set pen to paper against him, and during the whole time you are preparing your answer, you may commend him by earnest prayer to the Lord's teaching and blessing. This practice will have a direct tendency to conciliate your heart to love and pity him; and such a disposition will have a good influence upon every page you write.
If you account him a believer, though greatly mistaken in the subject of debate between you, the words of David to Joab concerning Absalom, are very applicable: "Deal gently with him for my sake." The Lord loves him and bears with him; therefore you must not despise him, or treat him harshly. The Lord bears with you likewise, and expects that you should show tenderness to others, from a sense of the much forgiveness you need yourself. In a little while you will meet in heaven; he will then be dearer to you than the nearest friend you have upon earth is to you now. Anticipate that period in your thoughts; and though you may find it necessary to oppose his errors, view him personally as a kindred soul, with whom you are to be happy in Christ forever.
But if you look upon him as an unconverted person, in a state of enmity against God and his grace (a supposition which, without good evidence, you should be very unwilling to admit), he is a more proper object of your compassion than of your anger. Alas! "He knows not what he does." But you know who has made you to differ. If God, in his sovereign pleasure, had so appointed, you might have been as he is now; and he, instead of you, might have been set for the defense of the gospel. You were both equally blind by nature. If you attend to this, you will not reproach or hate him, because the Lord has been pleased to open your eyes, and not his.
Of all people who engage in controversy, we, who are called Calvinists, are most expressly bound by our own principles to the exercise of gentleness and moderation. If, indeed, they who differ from us have a power of changing themselves, if they can open their own eyes, and soften their own hearts, then we might with less inconsistency be offended at their obstinacy: but if we believe the very contrary to this, our part is, not to strive, but in meekness to instruct those who oppose. "If peradventure God will give them repentance to the acknowledgment of the truth" (1 Tim 2:24-25). If you write with a desire of being an instrument of correcting mistakes, you will of course be cautious of laying stumbling blocks in the way of the blind or of using any expressions that may exasperate their passions, confirm them in their principles, and thereby make their conviction, humanly speaking, more impracticable.Read Newton's whole letter here.