“For this commandment which I command thee this day, it is not hidden from thee, neither is it far off...But the word is very nigh unto thee, in thy mouth, and in thy heart, that thou mayest do it.” – Deuteronomy 30:11-14

“The righteousness which is of faith, speaketh on this wise…The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach; that if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved.” – Romans 10:5-9

The gospel by no means involves itself in obscurity, but says, Believe and live, quite as distinctly as Moses said, “Do and live.” Here you have it, “Believe in the Lord Jesus Christ, and thou shalt be saved.” Moses’ utterance was single and by itself. He did not say, “Do and thou shalt live, and yet there is another way.” No; under the law it was nothing but “Do and live; leave undone and die.” So the gospel does not propose a second way, and suggest “a larger hope,” but it declares with solemn decision, “He that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.” It is just as clear as ever the law was, and quite as sharply distinct. Herein is no mystery: Jesus Christ came into the world to save sinners, and whosoever believeth in Him shall not perish, but have everlasting life. Wrapping everything up into one, the gospel says, “Trust thou in the Lord Jesus Christ, and thy sins shall be forgiven thee, and thou shalt be saved.” This believing, or trusting, is the whole of the matter, and neither heaven above nor the abyss beneath will ever reveal another salvation. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1700.cfm