Down-to-earth origin/ Etymology

I wonder why we say "down to earth" when referring to someone who is not deceitful. What's the logic behind it? I searched its origin but I didn't get much information. The origin only gives this:

Origin: This idiom may allude to angels or other celestial creatures coming down to Earth or being ‘cast down’ to join mortal humans. It also may derive from the notion of the word of God being ‘brought down to Earth’ so that humans could understand it.

Down-to-earth (adjective) meaning: If you say that someone is down-to-earth, you approve of the fact that they concern themselves with practical things and actions, rather than with abstract theories.

Could you please tell me why we use down-to-earth? Thanks a lot.

'Down to earth' as a journey from heaven

The notion of heaven as a paradise in the celestial region above earth and of earth as a place of travail beneath that region goes back very far indeed. Many early instances of "down to earth" in English publications thus refer to a physical (not abstract) relationship between the two. For example, from a 1592–1596 translation of St. Petrus Canisius, A Summe of Christian Doctrine:

FVLGENTIVS: MARY was made the windowe of heauen; because by her, god gaue the true light vnto the worlde. MARY was made the ladder of heauen; because by her God descended down to earth, that by her also men may ascend vnto heauen. MARY was made the restorer of women, because by her they are knowne to bee exempted from the ruine of the first curse.

From a 1597 translation of Gaspar de Loarte, Instructions and Aduertisements, How to Meditate the Misteries of the Rosarie of the Most Holy Virgin Mary:

And then maiest thou meditate, how his heauenly Father made him sitt in that high throne, on his right hand, where he nowe sitteth; that is to say, that he exalted him, & made him partaker of the greatest goodes and excellencies that either haue been, or euer shal-be graunted to any other; yea, that no creature can possiblye comprehend; which is to be vnderstood, in as much as he was man; for as he is God; he is equal with his Father, and hath the same glorye and Maiestie that his Father hath. The Prince of glorie being then receaued in this maner, (who neuer forgetteth them that loue and put their trust in him) sent two Angels down to earth to visite his most dear mother, with al the other companie, as also to informe them of his arriual into heauen, and to deliuer such other speeches as he spoke to them; by whose comming thou maiest wel weene, how great consolation they al receaued, what praises and what blessinges they yeelded vnto their Lord, whom after they had with most profound humilitie adored, they returned with great ioye to Ierusalem: ...

And from Andrew Willet, Limbo-mastix: That Is, A Canuise of Limbus Patrum Shewing by Euident Places of Scripture, Inuincible Reasons, and Pregnant Testimonies of Some Ancient Writers, That Christ Descended Not in Soule to Hell, to Deliuer the Fathers from Thence (1604):

It is denied, that Christs descension in soule to hell doth more set foorth his loue and fauour, than his crosse and passion. Saint Paul noteth this as the lowest point of Christs humiliation: He humbled himselfe, and became obedient vnto the death, euen the death of the crosse, Philip. 2. 8. And herein the Scripture doth chiefly set foorth the loue of Christ in dying for vs: who loued me, and gaue himselfe for me, saith S. Paul, Galath. 2. 20. And in this consisteth the fauour of our Prince, because hee hath visited from an high and redeemed his people, Luk. 1. 68. What greater fauour can we expect, than that our Prince should come from heauen down to earth, and vouchsafe to dwell among sinfull men, to die the death, and goe into the graue for them?

'Down to earth' (and 'down-to-earth') in a figurative or idiomatic sense

Today, the phrase "down to earth" has two distinct idiomatic senses, both of them surprisingly recent, according to Christine Ammer, The American Heritage Dictionary of Idioms, second edition (2013):

down to earth [1.] Back to reality. For example, It's time the employees were brought down to earth concerning the budget. P.G. Wodehouse had this idiom in Very Good Jeeves! (1930): "I had for some little time been living ... in another world. I now came down to earth with a bang." {Late 1920s} 2. Also, down-to-earth. Realistic or interested in everyday occurrences, as in She is a very down-to-earth person, not at all involved with the glamour of Hollywood. {1930s}

An Elephind newspapr database search turns up instances of hyphenated "down-to-earth," in the sense of "realistic or practical," from as early as 1911. Here are the earliest three instances.

From "The Croakers," in the [Salt Lake City, Utah] Godwin's Weekly (January 14, 1911):

The year 1911 gives great promise for substantial and steady growth. The past year has fully taught the lesson of readjustment that always follows abnormal business conditions, and the best informed business men are confident that the year beginning on a down-to-earth basis, the end of it will show an unusual record for general prosperity.

From an untitled item in the Breckenridge [Kentucky] News (November 6, 1912):

We could have written three or four columns about the missionary meeting, but one's words are limited on a country press. The services made us realize how great are the opportunities to learn, to serve, to give and to be happy right here in our own town. ... Men, women and children, who are in the Sunday Schools, the missionary auxiliaries, and in the societies of the young people, have possibilities undreamed of, and by work and sincere efforts may see them realized. The missionary meeting was full of down-to-earth ideas as well as inspiring thoughts and refreshings of love and faith.

And from "Vermont Boys and Corn," in the Burlington [Vermont] Weekly Free Press (May 1, 1913), reprinted from the Boston [Massachusetts] Transcript:

The country-life movement In Vermont Is a practical, down-to-earth affair. A State-wide esprit de-corps has been developed. In the farming communities and the villages, towns and counties the people are learning to co-operate. This co-operation is made possible by the various means by which the people are getting acquainted with one another, and by th enthusiastic leadership which gives expression to the common interests which exist in any community.

Instances of unhyphenated "down to earth" in the figurative sense of "back to reality" goes back to the 1830s, and may well be much older. For example, from an untitled item in the Staunton [Virginia] Spectator & General Advertiser October 18, 1838), reprinted from the Richmond [Virginia] Whig:

The world has not yet got upon the road to perfectability. It will, as it always has done, advance upwards for a season, and then fall back, whatever Godwinian enthusiasts may dream.—It has devised no scheme to absolve itself from the Malthusian principle, which, however high it may soar in anticipation, will still in the end drag it down to earth.

Likewise, from an [untitled item] in the [Melbourne, Victoria] Argus (June 3, 1880):

By way of showing that he lives as high up in the clouds as ever-as far removed from the dust and turmoil of practical politics-Mr. Wrixon gravely suggested that at this period of time it would be well to refer the reform question to a select committee. He had been charmed to hear the leaders on both sides declare that the matter should be treated apart from party considerations, and as Mr. Service had proposed some two years ago—when the Berry Government submitted its first bill—that the course he now recommends should be adopted, he does not despair of seeing his advice taken. Such pretty prattle in the coarse and matter of-fact arena of politics is quite touching. But even Mr. Wrixon would see, if he would come down to earth for a minute, that his desire to have the constitutional question settled by a nice little family party of disinterested politicians cannot be gratified. It is out of date, if nothing else stood in the way of attainment.

The sense of these figurative instances of "down-to-earth" and "down to earth" is that abstract thought and aspiration take us away from reality—to heaven, perhaps, or to Cloud Cuckoo Land—but that concern with practical action binds us to the real world. The figurative representation is thus not terribly different from the literal one that late-sixteenth-century authors envisioned between heaven and earth, except that it adopts very different implicit bias in favor of the latter.

Note, however, that the distinction generally doesn't address matters of honesty or sincerity versus deceit, but of practicality or realism versus fanciful thought.