Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant. Hebrews 9:15, ESV
The Gospel is explained very well in the New Testament. The role of the Church in the World from the time of Christ’s Ascension until His return is very well defined for us there as well (to go and make disciples from all the earth, teaching them to observe all that He taught). This Great Commission is not to ‘be the Gospel’ nor is it to ‘redeem the earth’ nor is it to ‘make the world a better place.’
No, it is to go and tell the Good News that we have a mediator of a new covenant and those who are called may receive the promised eternal inheritance that is made possible through the death of their Saviour who has redeemed through the shedding of His blood (see the passage above). You see, this is the Good News.
This is the Gospel. To preach Jesus is to preach what Christ has done not what people have done, not what churches have done, not what religion has done or what being religious can do for anyone. It is simply to proclaim this message relying on the power and work of the Holy Spirit to draw those called by God to saving faith. Let’s take a closer look at this “redemption.”