Excerpt from Good News article:
St. Valentine, Cupid and Jesus Christ
God warned ancient Israel, the people He chose to represent true religion, not to mix pagan customs with worshiping Him as the true God. "When the Lord your God cuts off from before you the nations which you go to dispossess, and you displace them and dwell in their land, take heed to yourself that you are not ensnared to follow them, after they are destroyed from before you, and that you do not inquire after their gods, saying, 'How did these nations serve their gods? I also will do likewise.' You shall not worship the Lord your God in that way; for every abomination to the Lord which He hates they have done to their gods ... Whatever I command you, be careful to observe it; you shall not add to it nor take away from it" (Deuteronomy 12:29-32).
Paul compares mixing paganism with Christianity to worshiping demons: "What am I saying then? That an idol is anything, or what is offered to idols is anything? Rather, that the things which the Gentiles sacrifice they sacrifice to demons and not to God, and I do not want you to have fellowship with demons. You cannot drink the cup of the Lord and the cup of demons; you cannot partake of the Lord's table and of the table of demons" (1 Corinthians 10:19-21).
Holidays such as St. Valentine's Day metamorphose into icons of Western culture, parodying religion. Most people don't care that its origins lie in the Roman Lupercalia and are rooted in tenets that have nothing to do with the Bible. It's this apathy about how to worship God, and the corresponding moral decay, that is the result of mixing Christianity with paganism.
Jesus said His followers would "worship the Father in spirit and truth" (John 4:23). Observance of this holiday is just one of many traditions that must be questioned if Christianity is to return to its foundation laid by Jesus Christ. GN
http://www.gnmagazine.org/issues/gn26/valentine.htm