Here is a statement of Ibn al-Qayyim (rahimahullah Ta'ala), which highlights the gravity of greeting the disbelievers on their celebrations. It has been extracted from a khutbah titled: Valentine's Day - A Pagan Practice by Abu Suhailah 'Umar Quinn.
The khutbah was delivered on the 13th of February 2014.
Abu Hurairah (رضي الله عنه) narrated that the Prophet ﷺ said: "Whoever invites to guidance will be rewarded like those who follow him in the guidance are rewarded, and that will not decrease the reward of those who have followed him." [Collected by Imām Muslim]
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