How is martial arts against God?

i am a Taoist-Christian (Christian with Taoist beliefs) and i do tae kwon do and practice in muay thai kickboxing , and i love it and look foward to becoming a black belt and getting better in other martial arts ,but some people on the internet say you "cant go to heaven if you do martial arts" , even though in the bible , God once said we must train our hands for war and sell our belongings to buy swords , and even God told david to KILL goliath so why do people exactly say i cant go to heaven because martial arts !!!!