Coming from a Christadelphian perspective the reaction to the Five Foundations I've just outlined might be "but it's not that simple!"
But it is. Here is how the Bible itself puts the Gospel in a nutshell:
What Jesus taught
From that time on Jesus began to preach, "Repent, for the kingdom of heaven is near." … Jesus went throughout Galilee, teaching in their synagogues, preaching the good news of the kingdom, and healing every disease and sickness among the people." (Matthew 4:17, 23)
"Jesus went through all the towns and villages, teaching in their synagogues, preaching the good news of the kingdom and healing every disease and sickness." (Matthew 9:35)
What Jesus told His disciples to teach
"He told them, "This is what is written: The Christ will suffer and rise from the dead on the third day, and repentance and forgiveness of sins will be preached in his name to all nations, beginning at Jerusalem" (Luke 24:46-47).
What the apostles taught
They [the Twelve] went out and preached that people should repent." (Mark 6:12)
"Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit" (Acts 2:38).
"First to those in Damascus, then to those in Jerusalem and in all Judea, and to the Gentiles also, I preached that they should repent and turn to God and prove their repentance by their deeds." (Acts 26:20)
"However, I consider my life worth nothing to me, if only I may finish the race and complete the task the Lord Jesus has given me - the task of testifying to the gospel of God's grace. Now I know that none of you among whom I have gone about preaching the kingdom will ever see me again." (Acts 20:24-25).
Yes, it really is that simple!
So here are the five foundations of the Gospel of the Kingdom (in a nutshell):
God is love and everything He does is driven by His love for His creation and His desire that everyone will be saved. The whole Bible is summed up in the commandment to love God and to love our neighbour, and we can love in this way because God first loved us, demonstrated His love to us, and showed us through Jesus how to love. Because of His love for us God wants to give us the Kingdom. It is His gift to us. There is nothing we can do to earn, merit or deserve it, but He gives it to us as His gift. All we need to do is turn to God, to repent or change our direction, and God will provide all the resources we need to live the Christian life, the kingdom life. When we turn to God He forgives us for everything we have done wrong in the past, and continues to forgive our mistakes and failings. When we turn to God we begin to live under His reign, under the authority or Lordship of Jesus, in the kingdom of God. As we grow by God's grace, with the help He provides, we develop a character which reflects His characteristics: we becoming loving, forgiving and gracious to other people, and we take the Good News of the Kingdom even further.