Transgress in verse 4 means rebellion, not just sin. God considered Israel's syncretistic approach to religion to be an outright rejection of His way of life.
Amos is speaking sarcastically when he suggests that the people sacrifice and tithe more often. "If you bring your tithes every three days (NKJV, AMP) instead of every three years," he says, "maybe your god, Baal, will respond." This sounds somewhat like Elijah's sarcastic comments in I Kings 18:27.
Amos mentions "leaven" in verse 5. Leaven was not allowed to be in any sacrifice: "No grain offering which you bring to the LORD shall be made with leaven, for you shall burn no leaven nor any honey in any offering to the LORD made by fire" (Leviticus 2:11). Only one offering, the wave loaves on Pentecost, was made with leaven (Leviticus 23:17). A sin offering preceded the offering of the wave loaves, the leavening in them representing the sins still in the congregation of Israel.
Here, Amos' sarcasm continues. The Israelites might as well have been making all their sacrifices with leaven because all their traditions, doctrines, customs, and religious duties were nothing but vanity. Even though they were sincere in doing them, they were nevertheless a leaven brought in from the world. In like manner, Jesus tells us to beware of the leaven of the Pharisees and Sadducees (Matthew 16:6-12), that is, of their doctrine and their traditions.
Even a quick glance at modern religious practices reveals how thoughtlessly people accept the doctrines and traditions they have learned—without proving them. Millions of sincere people attend church every week, celebrate the holidays, and send their children to church schools without ever proving their beliefs. They sing in the choir and donate generously when the plate is passed, but they do not really know—have an intimate relationship with—the god they worship. They just blindly accept the leaven they were taught while growing up.
John W. Ritenbaugh
Prepare to Meet Your God! (The Book of Amos) (Part Two)
Because of their connection to Israel's past, Bethel, Gilgal, and Beersheba all bore significant religious meaning to the common Israelite. Jeroboam I set up a golden calf at Bethel (I Kings 12:25-31), since the city had religious associations from the days of Jacob (Genesis 28:10-22; 35:1-7). Gilgal's significance sprang from Israel's entrance into Canaan after her forty years in the wilderness and the circumcision of her men there (Joshua 5:1-12). Beersheba had strong connections with Abraham, Isaac, and Jacob, the nation's fathers (Genesis 21:22-34; 22:19; 26:32-33; 28:10).
Even so, Israelite religion displeased God on two counts. First, the Israelites of Amos' day were guilty of following the sin of Jeroboam I, combining the worship of the true God with that of idols. God hates idolatry (Exodus 20:1-6). Apparently, the people were thronging to these pagan shrines and punctiliously offering sacrifices. In all their religious fervor, however, their eyes were not upon the God of heaven. Their religious practice was not done in obedience to God as they claimed, but had been conceived in the mind of a man. In His denunciations of their religion, God tells them that their worship would do them no good because its foundations were in a source other than Himself.
Second, their religion was self-pleasing. Because of their careful observance of their form of worship, Israelites felt good about themselves, but they forgot their social responsibility. They failed to love their neighbors (Amos 8:4). Ritual sexual indulgence was common practice (Amos 2:7). Despite their sincerity, they abandoned all godly standards and values and despised authority and law (Amos 3:10).
John W. Ritenbaugh
Prepare to Meet Your God! (The Book of Amos) (Part One)
Three cities of Israel had become religious centers and places of pilgrimage: Bethel, Gilgal and Beersheba. What is intriguing is that, even in their spiritual indifference, the Israelites loved to go to church! Since Amos indicates that their social lives may have revolved around the church, their purely social, not religious, motives may have been the problem.
This is intriguing in light of Laodiceanism. God says, "You may be coming to church regularly and enjoying it, but while you are there, you are sinning!" The scriptures are unclear about what the exact sins were. They may have been breaking the Sabbath somehow, or they may have been indifferent to the messages they heard. What their sins were makes no difference because God's judgment of their show of religion is that their hearts were not in it.
John W. Ritenbaugh
The World, the Church, and Laodiceanism