David, therefore, with the highest reason, declares, that although God should not speak a single word to men, yet the orderly and useful succession of days and nights eloquently proclaims the glory of God, and that there is now left to men no pretext for ignorance; for since the days and nights perform towards us so well and so carefully the office of teachers, we may acquire, if we are duly attentive, a sufficient amount of knowledge under their tuition.
There is no language nor speech [where] their voice is not heard. This verse receives two almost contrary interpretations, each of which, however, has the appearance of probability. As the words, when rendered literally, read thus — No language, and no words, their voice is not heard — some connect the third and fourth verses together, as if this sentence were incomplete without the clause which follows in the beginning of the fourth verse, Their writing has gone forth through all the earth, etc. According to them, the meaning is this:– The heavens, it is true, are mute and are not endued with the faculty of speech; but still they proclaim the glory of God with a voice sufficiently loud and distinct. But if this was David’s meaning, what need was there to repeat three times that they have not articulate speech? It would certainly be spiritless and superfluous to insist so much upon a thing so universally known. The other exposition, therefore, as it is more generally received, seems also to be more suitable. In the Hebrew tongue, which is concise, it is often necessary to supply some word; and it is particularly a common thing in that language for the relatives to be omitted, that is to say, the words which, in which, etc., as here, There is no language, there is no speech, [where  ] their voice is not heard.  Besides, the third negation, vly, beli,  rather denotes an exception to what is stated in the preceding members of the sentence, as if it had been said, The difference and variety of languages does not prevent the preaching of the heavens and their language from being heard and understood in every quarter of the world. The difference of languages is a barrier which prevents different nations from maintaining mutual intercourse, and it makes him who in his own country is distinguished for his eloquence, when he comes into a foreign country either dumb or, if he attempt to speak, barbarous. And even although a man could speak all languages, he could not speak to a Grecian and a Roman at the same time; for as soon as he began to direct his discourse to the one, the other would cease to understand him. David, therefore, by making a tacit comparison, enhances the efficacy of the testimony which the heavens bear to their Creator. The import of his language is, Different nations differ from each other as to language; but the heavens have a common language to teach all men without distinction, nor is there any thing but their own carelessness to hinder even those who are most strange to each other, and who live in the most distant parts of the world, from profiting, as it were, at the mouth of the same teacher.
4. Their writing has gone forth, etc. Here the inspired writer declares how the heavens preach to all nations indiscriminately, namely, because men, in all countries and in all parts of the earth, may understand that the heavens are set before their eyes as witnesses to bear testimony to the glory of God. As the Hebrew word qv, kav signifies sometimes a line, and sometimes a building, some deduce from it this meaning, that the fabric of the heavens being framed in a regular manner, and as it were by line, proclaims the glory of God in all parts of the world. But as David here metaphorically introduces the splendor and magnificence of the heavenly bodies, as preaching the glory of God like a teacher in a seminary of learning, it would be a meagre and unsuitable manner of speaking to say, that the line of the heavens goes forth to the uttermost ends of the earth. Besides, he immediately adds, in the following clause, that their words are every where heard; but what relation is there between words and the beauty of a building? If, however, we render qv, kav, writing, these two things will very well agree, first, that the glory of God is written and imprinted in the heavens, as in an open volume which all men may read; and, secondly, that, at the same time, they give forth a loud and distinct voice, which reaches the ears of all men, and causes itself to be heard in all places.  Thus we are taught, that the language of which mention has been made before is, as I may term it, a visible language, in other words, language which addresses itself to the sight; for it is to the eyes of men that the heavens speak, not to their ears; and thus David justly compares the beautiful order and arrangement, by which the heavenly bodies are distinguished, to a writing. That the Hebrew word qv, kav, signifies a line in writing,  is sufficiently evident from Isaiah 28:10, where God, comparing the Jews to children who are not yet of sufficient age to make great proficiency, speaks thus:
“For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little.”
In my judgment, therefore, the meaning is, that the glory of God is not written in small obscure letters, but richly engraven in large and bright characters, which all men may read, and read with the greatest ease. Hitherto I have explained the true and proper meaning of the inspired writer. Some have wrested this part of the psalm by putting upon it an allegorical interpretation; but my readers will easily perceive that this has been done without reason. I have shown in the commencement, and it is also evident from the scope of the whole discourse, that David, before coming to the law, sets before us the fabric of the world, that in it we might behold the glory of God. Now, if we understand the heavens as meaning the apostles, and the sun Christ, there will be no longer place for the division of which we have spoken; and, besides, it would be an improper arrangement to place the gospel first and then the law. It is very evident that the inspired poet here treats of the knowledge of God, which is naturally presented to all men in this world as in a mirror; and, therefore, I forbear discoursing longer on that point. As, however, these allegorical interpreters have supported their views from the words of Paul, this difficulty must be removed. Paul, in discoursing upon the calling of the Gentiles, lays down this as an established principle, that, “Whoever shall call upon the name of the Lord, shall be saved;” and then he adds, that it is impossible for any to call upon him until they know him by the teaching of the gospel. But as it seemed to the Jews to be a kind of sacrilege that Paul published the promise of salvation to the Gentiles, he asks whether the Gentiles themselves had not heard? And he answers, by quoting this passage, that there was a school open and accessible to them, in which they might learn to fear God, and serve him, inasmuch as “the writing  of the heavens has gone forth through all the earth, and their words unto the ends of the world,” (Romans 10:18.) But Paul could not at that time have said with truth, that the voice of the gospel had been heard through the whole world from the mouth of the apostles, since it had scarcely as yet reached even a few countries. The preaching of the other apostles certainly had not then extended to far distant parts of the world, but was confined within the boundaries of Judea. The design of the apostle it is not difficult to comprehend. He intended to say that God, from ancient times, had manifested his glory to the Gentiles, and that this was a prelude to the more ample instruction which was one day to be published to them. And although God’s chosen people for a time had been in a condition distinct and separate from that of the Gentiles, it ought not to be thought strange that God at length made himself known indiscriminately to both, seeing he had hitherto united them to himself by certain means which addressed themselves in common to both; as Paul says in another passage, that when God,
“in times past, suffered all nations to walk in their own ways, he nevertheless left not himself without a witness,”
(Acts 14:16, 17.)
John Calvin, Commentary on Psalm 19