Here is my opinion on the said question.
Firstly, the penalty for not spreading the gospel is Death, not just physical death, but spiritual death and it spread across all levels. Furthermore, if we don’t spread the gospel, we are at risk of loosing our treasures stored up in Heaven. According to
Colossians 1:5-6 which says
5 the faith and love that spring from the hope(A) stored up for you in heaven(B) and about which you have already heard in the true message(C) of the gospel (6) that has come to you. In the same way, the gospel is bearing fruit(D) and growing throughout the whole world(E)—just as it has been doing among you since the day you heard it and truly understood God’s grace.
But I don’t want dwell on the disadvantages of not spreading the Gospel but rather, I’ll like to speak about the enumerable benefits of spreading the gospel.
Evangelism deepens our understanding of the most fundamental truths of the scripture; When we teach something to someone, we are forced to have a deeper understanding of what we are teaching in order to teach it better and correctly, which in turns makes us grow in wisdom. When we spread the gospel, it grows our love for God and also our neighbors.
Evangelism protects us from mistakenly assuming that those around us are saved, we might know of a friend who often goes to church and is even a worker but has not yet accepted Christ, if we don’t reach out to speak more about the word that individual may never get saved and might be at the edge of death. Here is what the Bible have to say about those who live by the flesh and those who live by the Spirit.
5 Those who live according to the flesh have their minds set on what the flesh desires;(A) but those who live in accordance with the Spirit have their minds set on what the Spirit desires.(B) 6 The mind governed by the flesh is death,(C) but the mind governed by the Spirit is life(D) and peace. 7 The mind governed by the flesh is hostile to God;(E) it does not submit to God’s law, nor can it do so. 8 Those who are in the realm of the flesh(F) cannot please God.
The spirit that the above verse is talking about comes when you have accepted Christ as your Lord and Savior.
In conclusion, we should not wait to hear the disadvantages of not spreading the Gospel before we go into the world and preach, when we already know the benefits of The Good News, It is our solemn duty as Children of God to spread the gospel according to Mark 16:15 which says “And he said to them, “Go into all the world and proclaim the gospel to the whole creation”. If we fail to do that, we have strangled the Gospel at birth.