The Gospel IS an offence to those that are being lost but it is unto salvation for the believer. If I see someone heading toward destruction, I am expected to tell them so, if they believe me and turn away from that path of destruction, I have help save them, if I do not tell them, I can be held responsible for their lost soul, if they do not believe me and keep heading toward destruction, then I will not be held responsible.
There is a spiritual battle between good and evil for man's soul. Man is a sinner, and sin can not enter into Heaven. Sin must be paid for and only innocent blood can pay for sin. That is why Jesus, who is sinless came to earth to pay for all man's sin by taking those sins upon Him and paying for those sins through His blood being shed to pay that cost. But man must accept and believe this and accept Jesus as his redeemer, if he does not, than he is still responsible to pay for his own sins. No other religion offers a way or a Saviour that paid for man's sins. The bible tell the way man must go but unfortunately too many people do not read the bible, and the bible says man suffers for the lack of knowledge.