A one verse gospel?
In your quest to simplify the new covenant, don't exclude the words of Jesus. Jesus stated that those that didn't truly repent would perish. He said that you have to believe and be baptized to be saved. He said that you must be born of water and spirit to enter the kingdom. He said that all that believe like the scripture says should receive the Holy Ghost. He said that you must lay down your life and follow Him. He said that you can't look back or you are unfit for the kingdom. He said that you must endure to the end to be saved.
Don't forget the words of Peter in Acts 2. When he was asked by the crowd how they should respond to the gospel, he said repent, be baptized in Jesus name for the forgiveness of sins, and receive the Holy Ghost. He said save yourself from this wicked generation. Peter wrote that "baptism now saves us." He warned that it would be better for those that returned to their previous lives of sin to have never heard the gospel. He said that we should serve the Lord with reverent fear and be holy in our conduct because the day of judgment is coming.
Don't forget the words of Paul. He said that we put on Christ when we are baptized. He said that if we don't have the spirit of Christ than we are none of his. Paul told us that faith requires diligently seeking until we receive God's rewards. He proclaimed that we are saved by the washing of regeneration and the renewing of the Holy Ghost. He said that grace teaches us to live righteously. Assuming that Paul wrote Hebrews, he warned that those that willfully sin abuse grace and walk on the blood of the Son of God. He said that judgment starts at the house of the Lord and that saints that continued in sin would face God's wrath. He told us that no one would see the Lord without holiness.
We must believe in our hearts and confess with our mouths. We do this when we repent. We do this when we call on Jesus in baptism. We do this while seeking and receiving the Holy Ghost. We do this when praying for grace, strength, and forgiveness in our daily lives.
Make it plain, but don't leave part of it out. There is a new birth with a new heart, mind, and Spirit that is essential to being a new creature in Christ. Thank God for new life!
What should we say? Nothing less than the full council of God will do.
For more information regarding the full message of salvation: