** It is recommended you read understanding the forms of giving in church first
Tithing is clearly a Jewish tradition which according to Bible was commanded by God. It is more like their national taxation system.
Long before the establishment of the law, Abraham was cited paying tithe to Melchizedek in the Bible but according to history, it is the usual practice in the ancient kingdoms that were in the time of Abraham.
Tax is paid to the one that owns the law that governs you. Abraham had the opportunity to meet with the rulership of Salem (an unrecognized kingdom) who represented He that owned the law that governs the kingdom not built by men; he chose to pay the tithe of all things to him showing that that is where his real command comes from – this is a spiritual principle that can be tapped into. By virtue of it, God proved himself and took personal anything that concerned Abraham.
Why is it like robbing God?
God was saying, it was his entitlement. Israel is seen by God as His kingdom and therefore a portion of their harvest should come to Him because He was their King. Not paying tithe in those days can be likened to evading tax. He has His rod of punishment which are the devourers for anyone that refuses to pay. He also rewards anyone that pays with express access to His good hand (opening of the gate of heaven).
Tithing is a spiritual principle that can be tapped into. It is not an obligation to us (as we are not under the law), rather we can chose to make a statement with it after the order of Abraham constantly showing that we do not belong to this world acknowledging his ownership over our entire being will make him take whatever belonging to us as something personal to him.