The Lord had said, at the beginning of the cited ch. 24 of Exodus, to Moses: “Come up to the Lord, thou and Aaron, Nadab and Abihu, and seventy of the ancients of Israel.” Behold whom the Lord expressly indicates [that] he wills to accompany Moses [as he is] about to ascend the mount — and of Joshua he casts not even the least mention. How, then, does Moses lead him with [him]? Abulensis (q. 17) thinks it can be answered [that] our Joshua was in the number of those seventy elders. But he was then a youth, as [was] said above — or, what amounts to the same, a man still of flourishing age. And did we not see above (qq. 1 and 3) [that] he was reckoned among the young men, and set before all? Nor indeed would I believe [that] Moses wished to have any of the elders — [those] who were now holding magistracies in the commonwealth, who were set over the deciding of causes — on whose service he would wish to draw in the ministries of his chamber, his table, and the rest. That cause therefore pleases more which the same Abulensis subjoins: [that Joshua was] led by Moses as [his] minister, so that, namely, if matter or occasion should require, he might have his ministry at hand. For since (as [was] recited above from Aristotle, q. 1) “the servant is an animate instrument of the master” — his hand, as it were, foot, eye, or tongue — Joshua is now reckoned as the same and one [thing] with Moses. And therefore also [in] v. 2, when it is said, “and Moses alone shall come up to the Lord,” his own member or portion is not excluded from him — just as (1 Kings [3 Reg.] ch. 12, v. 20), when [it is said that], “except the tribe of Judah only,” none is handed down [as] having followed David’s house, and (2 Kings [4 Reg.] ch. 17, v. 18), when [it is said] “there remained only the tribe of Judah,” alone (Gk. μονωτάτη), the tribe of Benjamin is nevertheless not withdrawn from it, nor torn away. Nor, when “Tobias alone” is said to have gone to the temple of Jerusalem, are a few companions denied him, as I noted from his book, ch. 1. Moral and mystical reasons too can be brought — not altogether, as I think, discordant. The first: that the supreme dignity of Joshua might gradually become known, when, Aaron himself, the elders, this man’s sons Nadab and Abihu, [and] in short the seventy [chief] nobles of the whole people [being] left behind…
(col. 110)
…he alone is chosen [as] partner and companion in so wondrous an ascent. The second: since (as was said in the previous question) the name “Horeb” was put upon this mount for its dryness and aridity, and it was called “Sinai” for the brambles and thorns with which it bristled — it is at all events that steep, rough, and difficult mount of virtue, which the Gentile Hesiod too depicted, [which] Cebes set forth in his Tablet, [and which] Cicero commended (that young men might build [it] up, and always have it in [their] mouth — book 6, Ep. to Fam. 18). To this [mount], then, [being] about to ascend, why should that best of lords and teachers not take with him the best of all [his] ministers and disciples? Why should Joshua not join himself to him as eagerly as possible, [and] follow him? — so that, namely, all lords and masters may have [an example of] how to go before their own [people], their servants, or their disciples, and these [an example of] how they ought to follow and accompany them. To this mount Epicurus does not ascend; nor [do] those who chase after only gardens, springs, and shades — who say (Wisdom 2, v. 8): “Let us crown ourselves with roses before they wither; let there be no meadow which our luxury doth not pass through.” But Moses and Joshua — that is, noble hearts, and full of God — seek the parched crags of Horeb, the rough brambles of Sinai, because they understand [that] in these are virtue, duty, God’s Law, and God himself. The third: Joshua was to be Moses’s successor. And therefore it especially behooved him, both in himself and in the whole people, to press the divine Law, to make the most of it, [and] to set it before all things great and small. But by what means could so great a zeal, love, and admiration of that Law be better imprinted and stamped upon Joshua, than if he beheld with his own eyes how Moses prepared himself to receive it divinely, how he worshipped the Deity with most holy prayers, [and] how at last he was expressly called up by him into a loftier citadel, as it were, and a holier sanctuary, and was kept there so long, taught, and instructed? The fourth: just as Isaac, led onto the mount by [his] parent Abraham, was a type of those who would one day ascend Golgotha, the mount of Christ the Lord — so indeed our Joshua now signifies that same ascent of our Jesus. For truly Calvary was, to Christ Jesus, a most dreadful Horeb, a most holy Sinai — where he cried “I thirst,” and, crowned with thorns, poured out [his] blood and [his] life. And Joshua was led by Moses, because our Lord was dragged and hurried thither by the Synagogue itself, and by the priests and princes of the Mosaic people. The fifth, finally, cause can be that our Jesus too was to give his own Law and precepts on a mountain. For “seeing the crowds” (Matt. 5, v. 1), “he went up into a mountain; and when he was set down, his disciples came to him, and opening his mouth he taught them, saying: Blessed are the poor in spirit,” and what follows. Yet because these precepts are full of charity and grace, that difference must be observed which the Apostle notes (Heb. 12, v. 18): “For you are not come to a mountain that might be touched, but you are come to Mount Sion, and the city of the living God.” And these [were] the causes that now occurred [to me].
The ready and easy answer is [that] he ascended when Moses ascended — after the Law’s giving, receiving, and confirmation (taken from Exodus ch. 24) [and] ratified by the sacrosanct covenant. But other various little questions spring up.