The Present Day Kingdom
John the Baptist, who came before Christ, proclaimed the message, “Repent, for the kingdom of heaven is at hand!” Matthew 3:2
This Kingdom was to come in the first century with "power." Christ himself said, “Assuredly, I say to you that there are some standing here who will not taste death till they see the kingdom of God present with power.” Mark 9:1
According to Christ, this "power" was to also come with the Holy Ghost. “It is not for you to know times or seasons which the Father has put in His own authority. But you shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth.” Acts 1:8
This "power," that Christ said was coming with the Holy Ghost, came at the same time as the Kingdom, on the Day of Pentecost. When the Day of Pentecost had fully come, they were all with one accord in one place. And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. Then there appeared to them divided tongues, as of fire, and one sat upon each of them. And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance. Then those who gladly received his word were baptized; and that day about three thousand souls were added to them. And the Lord added to the church daily those who were being saved. Acts 2:1-4, 41, 47
The Kingdom was present during the days of Paul. He has delivered us from the power of darkness and conveyed us into the kingdom of the Son of His love, (Col. 1:13). We see here that the Kingdom is spoken of in present tense which means it had to already be in existence.
Christ also spoke of the Kingdom existing during Peter's life when He said to Peter, "And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it. And I will give you the keys of the kingdom of heaven, and whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” Matthew 16:18-19
With all the evidence God gave us through his word, you can only deduce that the Kingdom is the church and that exists today.