Romans 15.18For I will not venture to speak of anything except what Christ has accomplished through me to bring the Gentiles to obedience—by word and deed, 19 by the power of signs and wonders, by the power of the Spirit of God—so that from Jerusalem and all the way around to Illyricum I have fulfilled the ministry of the gospel of Christ; 20and thus I make it my ambition to preach the gospel, not where Christ has already been named, lest I build on someone else’s foundation, 21but as it is written,And Matthew 28:
"Those who have never been told of him will see,
and those who have never heard will understand."
Matthew 28.18 And Jesus came and said to them,
"All authority in heaven and on earth has been given to me.19 Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,20 teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age."
Now have a look at the Joshua Project and their information on unreached people groups. Christians, the mandate is ours!