70 But there is demanded from us an unwavering certainty. The Apostle expounding the whole secret of the Scripture passes on, Your word is near, in your mouth and in your heart. The words of our confession must not be tardy or deliberately vague: there must be no interval between heart and lips, lest what ought to be the confession of true reverence become a subterfuge of infidelity. The word must be near us, and within us; no delay between the heart and the lips; a faith of conviction as well as of words.
Heart and lips must be in harmony, and reveal in thought and utterance a religion which does not waver. Here too, as before, the Apostle adds the explanation of the Prophet's words, That is the word of Faith, which we preach; because if you shall confess with your mouth Jesus as Lord, and shall believe in your heart that God has raised Him up from the dead, you shall be saved. Piety consists in rejecting doubt, righteousness in believing, salvation in confessing. Trifle not with ambiguities, be not stirred up to vain babblings, do not debate in any way the powers of God, or impose limits upon His might, cease searching again and again for the causes of unsearchable mysteries: confess rather that Jesus is the Lord, and believe that God raised Him from the dead; herein is salvation.
What folly is it to depreciate the nature and character of Christ, when this alone is salvation, to know that He is the Lord. Again, what an error of human vanity to quarrel about His resurrection, when it is enough for eternal life to believe that God raised Him up. In simplicity then is faith, in faith righteousness, and in confession true godliness. For God does not call us to the blessed life through arduous investigations. He does not tempt us with the varied arts of rhetoric. The way to eternity is plain and easy; believe that Jesus was raised from the dead by God and confess that He is the Lord. Let no one therefore wrest into an occasion for impiety, what was said because of our ignorance. It had to be proved to us, that Jesus Christ died, that we might live in Him.
Source: On the Trinity (New Advent)