Just before Jesus was crucified, he met with his disciples to tell them the way to eternal life. He told them they needed to know God the way he knew God. He prayed that their spirits be "one in the same" with the spirit of God (the Holy Spirit) the same as his spirit was "one in the same" with the spirit of God. This is what Jesus was talking about when he said, "I and the Father are one." To be spiritually related to God through the Holy Spirit is the way God wants to "know" us and is the way to heaven. Jesus showed us that God gives the Holy Spirit to those who obey him. God will give us the Spirit the same way he gave it to Jesus if we obey him.
About 1, 500 years after Jesus told his disciples and those who were to follow (us) how to get to heaven, the Catholic Church had turned "Christianity" into something that wasn't even close to what Jesus was talking about. There were a number of "reformers" who risked being burned at the stake by going against the Catholic Church. Two of these men, Martin Luther and John Calvin (and others), came up with their own teachings that attracted followers and also led to the many different Protestant and Evangelical denominations we have today. The problem is none of these reformers taught what Jesus told his disciples. We still need a "final reformation."