Irenaeus

True knowledge, then, consists in the understanding of Christ, which Paul terms the wisdom of God hidden in a mystery, which "the natural man receiveth not," the doctrine of the cross; of which if any man "taste," he will not accede to the disputations and quibbles of proud and puffed-up men, who go into matters of which they have no perception. For the truth is unsophisticated; and "the word is nigh thee, in thy mouth and in thy heart," as the same apostle declares, being easy of comprehension to those who are obedient. …

Fragments from the Lost Writings of Irenaeus ↗

Peter of Alexandria

Moreover, we hear both also preaching, in the first place, not only repentance, but the kingdom of heaven, which, as we have learned, is within us; for the word which we believe is near us, in our mouth, and in our heart; which they, being put in remembrance of, will learn to confess with their mouths that Jesus is the Christ; believing in their heart that God hath raised him from the dead, and being as those who hear, that "with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation."

The Canonical Epistle ↗

Thomas Aquinas

Then when he says, but says the Scripture, he shows the fruit of faith on the same authority. First, he quotes the authority; second, he explains, at this is the word of faith; third, he proves that the explanation is fitting, at believe in your heart. First, therefore he says: but says the Scripture? It says this: the word is near you, even on your mouth and in your heart. For this is what follows after the aforementioned words in Deuteronomy: the word is very near you, in your mouth and in your heart (Deut 30:14). …

Commentary on Romans ↗