All founders of other religions (Islam, Buddhism, Mormon-ism, Hindus, Protestant dominations, etc) were men. NONE of them were able to perform miracles and thus validate their position as being sent by God to start a new religion much less as men worthy of any sort of credibility. None of them died for your sins or even suffered slightly for you. Only Jesus died for your sins so that you would know how to live (by following the teachings of the Church) and thus be happy and at peace regardless of personal circumstances. Only he founded God’s church which gives us the sacraments for the grace, that is the ability, to obey the commandments and follow Church teachings.
So people back in the first century knew that Jesus lived, was tortured to death and then an organization, the Catholic Church, with teachings on what to believe and how to live, that is, doctrine, appeared shortly after. The people who believed this oral word of God, that is the Gospel, (Mk 16:15-16) and were baptized were saved from their sins, that is, freed from the slavery to sin. Slavery to sin leads to misery, illness, ugliness, unnecessary suffering and often premature death. This is why Jesus came to earth, to ransom us from bondage to sin which leads to unhappiness (…the son of Man came not to be served, but to serve, to give his life as a ransom for the many). When we are free from sin we can then love God and our neighbor which leads to happiness and joy.
By practicing the faith the believers realized that when they followed the teachings of the ordinary and extra-ordinary magisterium (the pope and bishops in union with him) they were happy and society as a whole was better off. The poor and sick were taken care of, hospitals & orphanages founded, there was peace. Through the ages there has not been one contradiction in teaching. Atheists & theologians have claimed errors for centuries but so far no one has found one contradiction in Church teaching. That is because all teachings come from God who created us out of love and has given us the Church so we would know how to live (the light of Christ), be happy and at death receive eternal salvation in heaven. Would a loving father or mother tell his four year old to go play in the street? Of course not. It is the same with God our Father. Everything he gives us is for our own good and happiness.
Further proof what the Church teaches is true is by observing those who don’t follow the teachings. Many of them live lives of desperation, angry, unforgiving, gluttonous, miserable, enslaved to sex & have sexually transmitted diseases (know of any married couples who are faithful to each other with a STD?), in prison, drug addicts, in short they are enslaved to sin and can’t get out. The only way out is by following the teachings of the Catholic Church, which includes receiving the sacraments which will give them the grace (the ability) to change their lives. A prisoner (Russell Ford) in an Alabama state prison has been involved in the conversion of over 200 prisoners. Many have left after serving their time and only two (02) are recidivists. They say their daily prayers and go to Mass which gives them the grace to lead a normal, felony free life. No other religion or program has a 2% recidivism rate. I know someone in prison ministry who said the Protestant/Catholic ratio in town (about 20,000) is 50/50. Yet the Protestant prisoners number about 70%. Why is that? It is due to their teaching which states just accept Jesus as your Lord & Saviour & you will be “saved”. One does not have to obey the commandments or receive Jesus in the Holy Eucharist for the grace to help us obey the commandments. The result is that Protestantism leads to societal decay and disorder. This is how we know that everything God teaches us in the Catholic Church is true, by the effect it has on us.