"And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people." Matthew 4:23
The gospel of the kingdom. That's something to preach about. Jesus knew that if you would join with Him that what is in the kingdom (present and future) would be yours. The kingdom is the whole sphere of influence that God and His people have on the world. On this side of heaven the kingdom is made up of believers, the angels, the prayers of believers and the answers to those prayers from God (through people and angels and by the Holy Spirit.) The kingdom is in full operation today, on this earth. It is funded by God with all the necessary components needed and with the work of His people to shed light to whose living outside of the kingdom. That's the kingdom's purpose on this side of heaven; for believers to operate in and receive whatever is available and to provide light in the darkness for those who live outside the kingdom.
We are not in the kingdom to fight against those outside the kingdom, but to shed light on them so that they will recognize the Son of God through the interactive relationship that believers have with God that is conspicuous. Members of the kingdom are not to bash those outside the kingdom, but to invite them in. Neither are members of the kingdom to agree with those outside the kingdom, as if trying to bridge the gap between lost and found could be accomplished through agreement with false beliefs. Allowing those outside the kingdom the right to believe in any false thing they want to is not the same as acting like what they believe is the truth. Of course we are to be tolerant of other religions, their practices, their beliefs and their culture, but never in agreement. By agreeing that other religions have the same efficacy as Christianity is the same as enabling an alcoholic to continue to drink.
The kingdom of our God is open to anyone and everyone. The arms of Jesus are opened wide to any and all who will call on His name. God has empowered us to be light in a world where satan runs loose and sin permeates outside the kingdom walls and where sin is far too often let inside the kingdom walls by those who are not committed to a godly lifestyle.
It should be clear when those who have never heard of Jesus or maybe they are considering Jesus, there should be a clear line of demarcation set by those living for Jesus. But the line is not clear because so many Christians want to be called Christians, but live like the world. They confess Christianity, but they participate in many worldly pursuits. Too many Christians today are living lives that do not accurately represent "Christ in you, the hope of glory."
Jesus was preaching the kingdom when he was in Galilee and Jesus is preaching the kingdom today. Let us also preach/teach/live as if members of the kingdom. There never was a sister religion to Christianity.