In the book of Deuteronomy, it says we can't mix fabrics. Why is that?
Keep my statutes: do not breed any of your domestic animals with others of a different species; do not sow a field of yours with two different kinds of seed; and do not put on a garment woven with two different kinds of thread (Lev. 19:19).
You shall not wear cloth made from wool and linen woven together (Deut. 22:11).
This law applies only to the mixing of wool and linen. It does not apply to the mixing of any of materials. Jewish tradition tells us that the wool must be from a sheep or lamb. Camel wool, mohair, angora, cashmere, alpaca, or vicuna do not fall under this precept. The linen mentioned applies only to fibers from the flax plant and nothing else.
Jewish tradition has generally been perplexed by this prohibition. The great rabbis have called this law a chok. A chok is described as a law that a king has passed for his subjects, but they do not know why. For example, rabbis say they do not know why pork is forbidden or how a red heifer takes away sins, but they know that God has issued these commandments, and so they follow them, trusting in God’s wisdom.
Some theologians believe they have found the answer in the Bible where we are told that the high priest wore a garment of mixed wool and linen (Ex. 28:6-8, 39:4-5). It is believed that precisely because this was the garment of the high priest it was forbidden for the average person to wear. Clothing of mixed wool and linen was exclusively for the high priest; it was considered a consecrated kind of clothing. For the average person on the street to wear it would be akin to a modern person walking around in a clerical collar or Mass vestments.
We see something similar when it comes to the anointing oil used by the Israelites. The formula that was used was forbidden to be used for anything other than anointing oil (Ex. 30:31-38). The Jewish people were careful about making sure that things set aside for sacred use were not used for regular use.
In the modern day, the use of linen and wool in one garment would be extremely rare, so there’s little chance of violating this precept. However, since this is considered a ceremonial law and not a moral one, it would not apply to Christians.