Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
The Israelites were not only to suffer no idolatry to rise up in their midst, but in all their walk of life to show themselves as a holy nation of the Lord; and neither to disfigure their bodies by passionate expressions of sorrow for the dead (Deu 14:1 and Deu 14:2), nor to defile themselves by unclean food (vv. 3-21). Both of these were opposed to their calling. To bring this to their mind, Moses introduces the laws which follow with the words, "ye are children to the Lord your God." The divine sonship of Israel was founded upon its election and calling as the holy nation of Jehovah, which is regarded in the Old Testament not as generation by the Spirit of God, but simply as an adoption springing out of the free love of God, as the manifestation of paternal love on the part of Jehovah to Israel, which binds the son to obedience, reverence, and childlike trust towards a Creator and Father, who would train it up into a holy people. The laws in Deu 14:1 are simply a repetition of Lev 19:28 and Lev 21:5. למת, with reference to, or on account of, a dead person, is more expressive than לנפשׁ (for a soul) in Lev 19:28. The reason assigned for this command in Deu 14:2 (as in Deu 7:6) is simply an emphatic elucidation of the first clause of Deu 14:1. (On the substance of the verse, see Exo 19:5-6).
With reference to food, the Israelites were to eat nothing whatever that was abominable. In explanation of this prohibition, the laws of Lev 11 relating to clean and unclean animals are repeated in all essential points in vv. 4-20 (for the exposition, see at Lev 11); also in Deu 14:21 the prohibition against eating any animal that had fallen down dead (as in Exo 32:30 and Lev 17:15), and against boiling a kid in its mother's milk (as in Exo 23:19).
As the Israelites were to sanctify their food, on the one hand, positively by abstinence from everything unclean, so were they, on the other hand, to do so negatively by delivering the tithes and firstlings at the place where the Lord would cause His name to dwell, and by holding festal meals on the occasion, and rejoicing there before Jehovah their God. This law is introduced with the general precept, "Thou shalt tithe all the produce of thy seed which groweth out of the field (יצא construes with an accusative, as in Gen 9:10, etc.) year by year" (שׁנה שׁנה, i.e., every year; cf. Ewald, 313, a.), which recalls the earlier laws concerning the tithe (Lev 27:30, and Num 18:21, Num 18:26.), without repeating them one by one, for the purpose of linking on the injunction to celebrate sacrificial meals at the sanctuary from the tithes and firstlings. Moses had already directed (Deu 12:6.) that all the sacrificial meals should take place at the sanctuary, and had then alluded to the sacrificial meals to be prepared from the tithes, though only causally, because he intended to speak of them more fully afterwards. This he does here, and includes the firstlings also, inasmuch as the presentation of them was generally associated with that of the tithes, though only causally, as he intends to revert to the firstlings again, which he does in Deu 15:19. The connection between the tithes of the fruits of the ground and the firstlings of the cattle which were devoted to the sacrificial meals, and the tithes and first-fruits which were to be delivered to the Levites and priests, we have already discussed at Deut 12. The sacrificial meals were to be held before the Lord, in the place where He caused His name to dwell (see at Deu 12:5), that Israel might learn to fear Jehovah its God always; not, however, as Schultz supposes, that by the confession of its dependence upon Him it might accustom itself more and more to the feeling of dependence. For the fear of the Lord is not merely a feeling of dependence upon Him, but also includes the notion of divine blessedness, which is the predominant idea here, as the sacrificial meals were to furnish the occasion and object of the rejoicing before the Lord. The true meaning therefore is, that Israel might rejoice with holy reverence in the fellowship of its God.
In the land of Canaan, however, where the people would be scattered over a great extent of country, there would be many for whom the fulfilment of this command would be very difficult-would, in fact, appear almost impossible. To meet this difficulty, permission was given for those who lived at a great distance from the sanctuary to sell the tithes at home, provided they could not convey them in kind, and then to spend the money so obtained in the purchase of the things required for the sacrificial meals at the place of the sanctuary. ממּך ירבּה כּי, "if the way be too great (too far) for thee," etc., sc., for the delivery of the tithe. The parenthetical clause, "if Jehovah thy God shall bless thee," hardly means "if He shall extend thy territory" (Knobel), but if He shall bless thee by plentiful produce from the field and the cattle.
"Turn it into money," lit., "give it up for silver," sc., the produce of the tithe; "and bind the silver in thy hand," const. praegnans for "bind it in a purse and take it in thy hand...and give the silver for all that thy soul desireth, for oxen and small cattle, for wine and strong drink," to hold a joyous meal, to which the Levite was also to be invited (as in Deu 12:12, Deu 12:18, and Deu 12:19).
Every third year, on the other hand, they were to separate the whole of the tithe from the year's produce ("bring forth," sc., from the granary), and leaven it in their gates (i.e., their towns), and feed the Levites, the strangers, and the widows and orphans with it. They were not to take it to the sanctuary, therefore; but according to Deu 26:12., after bringing it out, were to make confession to the Lord of what they had done, and pray for His blessing. "At the end of three years:" i.e., when the third year, namely the civil year, which closed with the harvest (see at Exo 23:16), had come to an end. This regulation as to the time was founded upon the observance of the sabbatical year, as we may see from Deu 15:1, where the seventh year is no other than the sabbatical year. Twice, therefore, within the period of a sabbatical year, namely in the third and sixth years, the tithe set apart for a sacrificial meal was not to be eaten at the sanctuary, but to be used in the different towns of the land in providing festal meals for those who had no possessions, viz., the Levites, strangers, widows, and orphans. Consequently this tithe cannot properly be called the "third tithe," as it is by many of the Rabbins, but rather the "poor tithe," as it was simply in the way of applying it that it differed from the "second" (see Hottinger, de decimies, exerc. viii. pp. 182ff., and my Archol. i. p. 339). As an encouragement to carry out these instructions, Moses closes in Deu 14:29 with an allusion to the divine blessing which would follow their observance.