John Owen wrote, "Take away the Spirit from the gospel, and you render it a 'dead letter' of no more use to Christians than the Old Testament is to the Jews." He understood that the gospel is in power through the Holy Spirit. And, without the power of the Holy Spirit, the gospel changes no one.
The gospel of Jesus Christ is our salvation. The apostle Paul defined the gospel in these words, "For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek" (Romans 1:16). That is the gospel of Christ for everyone to hear, but saving only those who believe. They experience the power of God through the gospel changing their lives.
Christians are to spread the gospel for all to hear. The preacher proclaims the gospel to one and all without exception. That's our responsibility to the command of Christ (Matthew 28:19; Mark 16:15). Having obeyed the command of Christ, we leave the results to God. The gospel is the power of God, through the Holy Spirit convicting and drawing people to Christ (John 16:8-11).
It is not our methods nor our schemes that makes the gospel effective. Without the power of the Holy Spirit effectively drawing people to Christ, all is vain. That is no excuse to disobey the command to spread the gospel. It is however to make us constantly aware that only God gives the increase (1 Corinthians 3:7). The Holy Spirit is the gospel in power!