In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Imaam Muhammad Ibn Saaleh Al-Uthaymeen [rahimahullaah] said:
The heart of the caller [i.e. the caller to Islaam upon sound knowledge, clear-sightedness, wisdom, etc] should be relaxed towards the one who opposes him, especially if he knows that the one who opposes him has a good intention and has not opposed him except to establish the evidence he possesses; therefore, it is obligated on a person to be flexible in these affairs. He should not make this differing a cause for enmity and hatred, except against a man who opposes the [truth] obstinately after the truth has been made clear to him, but he carries on upon his falsehood; then indeed it is obligatory to deal with him [in a manner] he deserves-by alienating him and warning the people about him- because his enmity manifested after the truth has been made manifest to him and [refuses] to follow it. [1]
[1] [Source: (Zaad Ad-Daa’iyah Ilal laah’ page 27). Slightly paraphrased]
Articles to detail the above affair:
http://www.salaficentre.com/2015/05/a-misunderstanding-regarding-the-statement-laa-inkaar-fee-masaail-al-khilaaf-there-should-be-no-repudiation-in-the-affairs-of-khilaaf-i-e-differing/
http://www.salafitalk.net/st/viewmessages.cfm?Forum=9&Topic=12294