What is the "kingdom of God"?
Throughout the gospels Jesus refers to the kingdom of God several times (more than 100 times, in fact). And in Luke 9.2 Jesus sends his 12 apostles out specifically to "declare the kingdom of God." Bible scholars have pondered the exact nature of what the kingdom of God actually refers to, and there are many nuanced interpretations that remain today. As I see it, the kingdom of God represents the new reality brought forth by Jesus through his life, death, and resurrection. Jesus came to destroy the works father devil that first began in the Garden of Eden, when Adam and Eve fell into sin. He came to redeem people from the effects of living in that fallen world, and to usher in a new reality of atonement, forgiveness, and righteousness. Thus, when the 12 are sent out to "declare the kingdom of God," they are telling people that the destruction brought about by sin will be/has been remedied by the entrance of the Messiah onto the scene. Jesus has come, and he will right the wrongs caused by sin and build a new kingdom of righteousness.
This kingdom is partially realized when we put our trust in Christ. When we are saved from the consequences of sin and enter into the eternal life that God has prepared for those who trust in Christ, we become partakers (citizens) of this new kingdom. We no longer live in a world where the eternal effects of sin are hanging over our head. Instead, we live in a kingdom that is ruled by the righteousness of God in Christ, and we look forward to the full realization of that kingdom in this world when Jesus comes back. Until then, Jesus builds his kingdom in the hearts and lives of those who will be his subjects.
Can we be witnesses for Jesus by how we live?
Yes. The Bible clearly teaches that there is a marked difference between those who are living in the kingdom of God and those who are living outside of it (see Matthew 5.1-12, for example). And when the world sees us living as citizens of the kingdom of God, they take notice. They realize that we are different (Matthew 5.13-16). Moreover, 1 Peter 3 says that wives are to win over their unbelieving husbands through their godly behavior. So according to these scriptures and many more, we can be faithful, obedient witnesses for Jesus by exhibiting godly behaviors, actions, and attitudes for the rest of the world to see.
But it is important to note that this is only one part of our witness and/or testimony about the truth of the gospel. The New Testament also clearly and explicitly says that faith comes by hearing, not by seeing. In order for the message of the gospel to be communicated, it must be spoken. After all, it would be difficult to "live out" the reality of the kingdom of God described above. What kind of actions would you perform to communicate that the Messiah has come to rescue fallen sinners? In order to communicate this message, we must speak. The fruit of transformed lives and hearts bears witness to the truth of the gospel, but it does not explain the gospel. In order to declare the gospel, we must speak.
The disciples worked powerful miracles when they preached the gospel. Why don't we see those same kinds of miracles today?
Luke 9.1 says that when Jesus sent the 12 out to declare the kingdom of God, he also gave them "power and authority over all demons and to cure diseases." The reason the disciples had this power was not to wow the crowds with their abilities or to perform magic tricks for entertainment purposes, but to act as signs about the truth of their message. Remember, they were sent to "declare the kingdom of God" - this new reality that was being ushered into the world through the life, death, and resurrection of Jesus. The proof of this new reality was that the disciples had authority over demons and the power to heal diseases. Jesus didn't give the disciples power for the sake of them being able to do cool miracles, but for the sake of authenticating their verbal message.
It is my belief that we don't see these kinds of miracles accompanying the declaration of the gospel today because we don't need to see them - we've seen them already. The authenticating signs and wonders performed by the apostles prove to us - just as much as it did to the people who saw them - that the new reality of the kingdom of God in the hearts and lives of people who follow Jesus is actual, and that it is true. To require additional signs and wonders on top of the ones already given to us as proof seems to me to be redundant.
That being said, the power of God is still evident in his word when it is declared and shared. It brings the power of conviction, repentance, faith, obedience, and a host of other actions that are simply impossible for sinful human beings to perform. We cannot respond to the truth of God's word without his power to strengthen us to turn from sin and believe. We cannot obey God's word without the power of his Holy Spirit to empower our obedience. We cannot join God in his mission to declare his kingdom without his power to energize our efforts and strengthen us to care for those who are perishing. God's word today brings with it no less power than it did in the first century. That power just doesn't manifest itself in signs and wonders anymore.
Do we need to ask for God's power, or do we have it automatically?
All those who are trusting in Christ are empowered by the Holy Spirit to accomplish whatever it is that God has called them to do. This power is given to us at the time of our conversion. The Holy Spirit empowers us to combat sin in our lives, obey God's word, venture into ministry endeavors, and a host of other activities.
As believers, this power is available to us on demand. It does not require a special prayer or incantation in order for it to be accessed. It is not forced upon us, however. For example, although Christians have the power to battle against sin and temptation in our lives, there are many times when we neglect to access this power, and instead give into sinful temptations. When this happens, it is not that we do not have the power to resist temptation, but rather that we have neglected to use it. We are not slaves to our sinful nature, and we do not have to obey it. We have power over it, and a free will to refuse its enticing demands. This ability only exists because of the power of God. Yet, there are many times when we choose to not exercise or take advantage of this God-given power, because we still struggle against our flesh.
