The most important message of Jesus Christ is the announcement of the coming of the Kingdom of God (the Kingdom of Heaven). The Kingdom of God is not of this world, it is not material but spiritual and it is power, righteousness, peace and joy. It is a kingdom where God's rule is realized perfectly in all areas of life. Its founder and king is Jesus Christ Whose earthly ministry's aim was the extension of the Kingdom of God.
Miracles and signs proved and prove the coming of the Kingdom of God today, too. Jesus summed up the constitution and fundamental law of this Kingdom first in the "Sermon on the Mount". He revealed its internal truth and nature in His parables.
His acts and miracles - healing the sick, raising the dead, cleaning the lepers and driving out demons - proved supremacy and victory of the Kingdom of God over the kingdom of Satan and that nothing can stop its expansion and increasing.
The main instruction of Jesus Christ having been given to his disciples is to preach the gospel of the Kingdom of God and to present its spiritual strength and power. This commandment is just as valid today and is the main task of the Church.