"From that time Jesus began to preach and to say, "Repent, for the kingdom of heaven is at hand" (Matthew 4:17).
This is how Jesus started His ministry. It is the central theme of all His message, which even John the Baptist followed throughout his life.
Repent.
It is a true message then, and it is a true message now.
Repent means to turn to God and forsake (or leave behind) the life of sin (unrighteousness, wickedness, disobedience to God, or even trusting self). Of ourselves, there is no good deed that is fit to earn the life that God desires that we have with Him and the promise of eternal life. This is because God's standards of good are far above anyone in this world can do for themselves. He alone is good and fulfills those standards of good, making Him Holy.
Yet, He extended His goodness toward humankind by showing us a way that will allow us to have life, a meaningful one, as He originally intended, with Him. He gave us Jesus, who is God Himself in the form of man, who walked on the earth among humankind without sin. He was an example to us on how to live our lives in this world (fully trusting in God and doing His will) and, in the end, being the necessary offering to God so that life would be extended to all humankind who would believe in the work that Jesus did. Jesus died as an offering to free humankind from the bondage of death, and, thus, promising & giving eternal life.
Humankind was in bondage to death because it gave their authority to satan (who deceived them) to rule over the world and, thus, bringing death into the world. God, in the form of Jesus, died and fought back for that authority and left satan without any power. That recovery of power and authority, then, was sealed and ensured with the resurrection (or bringing back to life) of Jesus.
Jesus has full power and authority to rule the world, alongside those who believe in Him and His work (because He extended this to believers as well). Yet, the ruling of the world is not conventional - meaning it is one that is unseen and only witnessed through the life that a believer lives, which follows after the life that Jesus lived on the earth.
Furthermore, this recovery of power and authority means that the kingdom of heaven (God's kingdom) is around us and within each believer in Jesus.
Does this mean that God never had full power or authority? No. God always had the power and authority because He is the Creator of everything that is seen and unseen. The purpose was to provide a way that would bring humankind into having a relationship with God Himself, and have Him live through us. It just needed the proper time and place to occur, and it covered even those who followed Him sincerely and full-heartedly before Jesus Christ.
Back to the message,
Repent, and turn to God, for the kingdom of heaven is at hand. He gives life in the now and will give it in the afterlife to those who believe in Jesus.
Persevere and endure until the end.