(3) Your eyes have seen.--Literally, your eyes are they that see--i.e., you are witnesses of these things. The men who perished by the plague because of the iniquity of Beth peor--to the number of 24,000--seem to have been all members of the younger generation; for they had already passed the brook Zered. (See on Deuteronomy 2:13.)

Verses 3, 4. - The people had had personal experience of the danger, on the one hand, of transgressing, and the benefit, on the other, of keeping God's Law; they had seen how those who sinned in worshipping Baal-peer were destroyed (Numbers 25:3, 9), whilst those who remained faithful to the Lord were kept alive. This experience the people had had only lately before, so that a reference to it would be all the more impressive. Baal-peor, the idol whose cultus was observed at Peor. Baal (Bal, Be cf., Bel, Lord) was the common name of the supreme deity among the northern of the Semitic-speaking people, the Canaanites, the Phoenicians, the Aramaeans, and the Assyrians. There were thus many Baals. Followed: walked after; a common Biblical expression for religious adherence and service (cf. Jeremiah 8:2; Jeremiah 9:14; and with a different formula, Numbers 32:12; Deuteronomy 1:36; Joshua 14:8; Judges 2:12, etc.). Ye that did cleave unto Jehovah your God. "To cleave unto one" is expressive of the closest, most intimate attachment and communion (cf. Genesis 2:24; Isaiah 14:1). The phrase is frequently used of devotion to the service and worship of the true God (cf. Deuteronomy 10:20; Joshua 22:5; Joshua 23:8; Acts 2:23, etc.); here it expresses the contrast between the conduct of those who remained faithful to Jehovah and those who forsook him to worship Baal. Are alive every one of you this day. "Thus they that keep themselves pure in general defections, are saved from the common destruction (Ezekiel 9:4-6; 2 Timothy 2:19; Revelation 20:4)" (Ainsworth).

Matthew Henry's Concise Commentary

4:1-23 The power and love of God to Israel are here made the ground and reason of a number of cautions and serious warnings; and although there is much reference to their national covenant, yet all may be applied to those who live under the gospel. What are laws made for but to be observed and obeyed? Our obedience as individuals cannot merit salvation; but it is the only evidence that we are partakers of the gift of God, which is eternal life through Jesus Christ, Considering how many temptations we are compassed with, and what corrupt desires we have in our bosoms, we have great need to keep our hearts with all diligence. Those cannot walk aright, who walk carelessly. Moses charges particularly to take heed of the sin of idolatry. He shows how weak the temptation would be to those who thought aright; for these pretended gods, the sun, moon, and stars, were only blessings which the Lord their God had imparted to all nations. It is absurd to worship them; shall we serve those that were made to serve us? Take heed lest ye forget the covenant of the Lord your God. We must take heed lest at any time we forget our religion. Care, caution, and watchfulness, are helps against a bad memory.

Gill's Exposition of the Entire Bible

Your eyes have seen what the Lord did because of Baalpeor,.... Because of the idolatry the people of Israel fell into by worshipping that idol, being drawn into it by the daughters of Moab and Midian, through the counsel of Balaam, with whom they committed fornication; which led them to the other sin, and both highly provoking to God. The Targums of Onkelos and Jonathan are,"what the Word of the Lord has done to the worshippers of the idol Peor;"

for all the men that followed Baalpeor, the Lord thy God hath destroyed them from among you; 24,000 persons died on that account; which being a recent thing, fresh in memory, and what they were eyewitnesses of, was a caution to them to avoid the same sins, as it is to us on whom the ends of the world are come, Numbers 23:9.

OpenBible.com