By believing in monetary tithing today to be collected by someone who is not from sons of Levi (Numbers 18:21). You’ve changed the commandment of God of what to tithe or give in 10% and to who shall it be given thus, it contradicts that God’s word is absolute.
Tithing in Lev 27:30 says, it belongs to God. He owns it, that is why, if you choose to redeem it, you’ll pay its corresponding value plus additional 20% of its value as according to Lev 27:31. To date, God who was revealed to be in the name of Jesus is alive, the owner of the tithes under the law is still alive and, the living owner says, give what belongs to Him to Levites. Yet, it is now being collected by someone who is not related to Levites. Did God change already His instruction of who shall collect His tithe? Because, if it’s still the same to date, a person who collect someone’s property who is still living, that person might be accused of thieving.