Am I sinful for providing internet access to my Christian neighbors and some irreligious youth, knowing that they use it for religious matters (the Gospel) or other matters unknown to me?

If it is highly probable that they will use it for something Allah has forbidden, then it must be withheld from them. Otherwise, the person would be sinful for assisting them in sin and transgression, in accordance with the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." One should seek Allah's help and not compromise on truth. Remember that whoever seeks Allah's pleasure despite people's displeasure, Allah will suffice him against the burden of people, and whoever abandons something for the sake of Allah, Allah will compensate him with something better.