Plural declension of οὐρανός in Matt. 16:18 v. singular declension of οὐρανός in Matt. 18:18

Question

In Matt. 16:19 where the Lord Jesus Christ bestows the keys of the kingdom of Heaven upon the apostle Peter, the Greek text uses the plural declension of οὐρανός, as follows:

καὶ δώσω σοι τὰς κλεῖς τῆς βασιλείας τῶν οὐρανῶν καὶ ὃ ἐὰν δήσῃς ἐπὶ τῆς γῆς ἔσται δεδεμένον ἐν τοῖς οὐρανοῖς καὶ ὃ ἐὰν λύσῃς ἐπὶ τῆς γῆς ἔσται λελυμένον ἐν τοῖς οὐρανοῖς ^{TR, 1550}

However, in Matt. 18:18 where keys are bestowed upon the remainder of the apostles, the Greek text uses the singular declension of οὐρανός, as follows:

Ἀμὴν λέγω ὑμῖν ὅσα ἐὰν δήσητε ἐπὶ τῆς γῆς ἔσται δεδεμένα ἐν τῷ οὐρανῷ καὶ ὅσα ἐὰν λύσητε ἐπὶ τῆς γῆς ἔσται λελυμένα ἐν τῷ οὐρανῷ ^{TR, 1550}

What significance, if any, is there in the usage of the plural versus the singular declension of οὐρανός in two verses which seem to be referencing the same authority (i.e., binding and loosing) bestowed upon the apostles by the Lord Jesus Christ?

_{Disclaimer: I reviewed the NA28 of Matt. 16:19 and Matt. 18:18 and did not notice any textual variants concerning οὐρανός in both verses.}

Answer 1

I found the contextual explanation given by Jonathan T. Pennington in Heaven and Earth in the Gospel of Matthew to be the most compelling answer to this conundrum. You can access an online version at this link and skip to pages 147-150. The author's summary of this section is offered below.

Matthew’s frequent use of the plural forms of does not stem from the inuence of Semitic morphology nor from a multiple heavens cosmology, but instead, he has intentionally used both singular and plural forms in an idiolectic pattern. Matthew inherited a linguistic world where singular forms of were by far the most common and were used in two different senses (for the visible world and the invisible/divine), and where heaven (sg) and earth was a stock phrase. He used the semantic exibility of and formed an idiolectic way of speaking in which he typically uses the singular forms for the one semantic pole of (the visible, earthly realm) and the plural for the other (invisible, divine realm), all the while retaining the traditional singular heaven and earth phraseology. While there are a few anomalies in this formulation, in most instances, other overriding factors can be seen to explain the aberrations. Even the three texts which appear anomalous make up only four of the eighty-two occurrences of, the rest of which all accord with the pattern. Whether Matthew borrowed this pattern from other literature such as Wisdom of Solomon or shared a common source with Testament of Abraham is unclear. Either way, the usage is far more developed and thematic in Matthew than anywhere else. Most importantly, without exception, Matthew never uses singular forms of in connection with the Father or the kingdom. This uncommon use of the plural to refer to the divine would likely catch the ear of the hearers and highlight and heighten the distinction between God and the world that the Evangelist is attempting to communicate. Matthew’s use of the uncommon plural forms reects not a linguistic nor a cosmological source, but a rhetorical purpose. He develops this idiolectic use of singular and plural forms for a literary and theological purpose: to contrast the heavenly realm with the earthly.