Let us first read the scripture again in Numbers 12:1-15: “Miriam and Aaron began to talk against Moses because of his Cushite wife, for he had married a Cushite. ‘Has the LORD spoken only through Moses?’ they asked. ‘Hasn't he also spoken through us?’ And the LORD heard this. (Now Moses was a very humble man, more humble than anyone else on the face of the earth.) At once the LORD said to Moses, Aaron and Miriam, ‘Come out to the Tent of Meeting, all three of you.’ So the three of them came out. Then the LORD came down in a pillar of cloud; he stood at the entrance to the Tent and summoned Aaron and Miriam. When both of them stepped forward, He said, ‘Listen to my words: When a prophet of the LORD is among you, I reveal myself to him in visions, I speak to him in dreams. But this is not true of my servant Moses; he is faithful in all of my house. With him I speak face to face, clearly and not in riddles; he sees the form of the LORD. Why then were you not afraid to speak against my servant Moses?’ The anger of the LORD burned against them, and he left them. When the cloud lifted from above the Tent, there stood Miriam — leprous, like snow. Aaron turned toward her and saw that she had leprosy; and he said to Moses, ‘Please, my lord, do not hold against us the sin we have so foolishly committed. Do not let her be like a stillborn infant coming from its mother's womb with its flesh half eaten away.’ So Moses cried out to the LORD, ‘O God, please heal her!’ The LORD replied to Moses, ‘If her father had spit in her face, would she not have been in disgrace for seven days? Confine her outside the camp for seven days; after that she can be brought back.’ So Miriam was confined outside the camp for seven days, and the people did not move on till she was brought back.”
Let’s examine the issue.
Moses, a Jew, apparently married a black African. We learn in Numbers that "Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married, for he had married a Cushite woman" (Num. 12:1). A Cushite is from Cush, a region south of Ethiopia, where the people are known for their black skin. We know this because of Jeremiah 13:23: "Can the Ethiopian [the same Hebrew word translated "Cushite" in Numbers 12:1] change his skin or the leopard his spots? The Jewish historian Josephus adds the detail that her name was Tharbis and she married Moses when he led the Egyptian forces on a military campaign to Ethiopia before embracing his Hebrew heritage and needing to flee to Midian. At the time of the marriage, neither Moses, nor Tharbis practiced the religion of the Hebrews. Nonetheless, the marriage was approved by God.
In response to Miriam’s criticism, God does not get angry at Moses; he gets angry at Miriam. The criticism has to do with Moses’ marriage and Moses’ authority. The most explicit statement relates to the marriage: "Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married, for he had married a Cushite woman." Then God strikes Miriam with leprosy. Why? Consider this possibility. In God’s anger at Miriam, Moses’ sister, God shows Miriam that although she has light skin that there is a skin much whiter and that skin can be so white that it is a decay. So we read, "When the cloud removed from over the tent, behold, Miriam was leprous, like snow" (Num. 12:10)
God does not say a critical word against Moses for marrying a black Cushite woman. It might be reasonable to think it was because Moses married the Cushite woman before he was called by God to deliver the children of Israel from bondage in Egypt.
But when Miriam criticizes God’s chosen leader, God strikes her skin with white leprosy. If you ever thought black was a biblical symbol for uncleanness, be careful; there is a worse white uncleanness also represented in the Bible, it is leprosy.
It is to be noted that God can never be wrong. He is the Creator of us all. When He chose Moses He already knew his marital status and yet He did not rebuke Moses for it. He accepted Moses’ marriage as a legal one although the woman was not a Hebrew. Miriam is not above God and had no standing to challenge the leader He chose to lead the Nation of Israel.