Malachi 3:8 not only proves that it is possible to rob God, but also the fact that some do. I believe “bring your tithes into the storehouse” was simply admonishing the people to give as they were commanded. One could rob God without taking anything out of the storehouse when they refused to put into the storehouse what was His. What Achan “stole” from God was never put into the treasury as God commanded but privately kept for his own benefit (Josh 6:19; 7:1,11,21). Achan sinned in this and suffered severely as a result (Josh. 7:20-26). It likewise follows that one could “rob” God when they refused to give into the treasury/storehouse as God commanded and would suffer consequences. “Ye are cursed with a curse: for ye have robbed me, even this whole nation” (Mal. 3:9). Neither would God accept an unworthy offering even from the hands of the priests (Lev. 22:19-25; Mal. 1:6-14). The point is, one could not just offer anything to God and it would be accepted. All were commanded to rightly give to God if they had it.
Today, all Christians are commanded to bring an offering to God to be put into the treasury/storehouse of the Lord on every first day of the week/Sunday “as God has prospered them” (1 Cor. 16:1-2). We must take a generous amount of what we have been blessed with right off the top and set it aside through the week to be given on the Lord’s Day/Sunday (2 Cor. 9:6-8). We must do this every week. If we are unable to come to worship due to sickness, travel, or our work schedule, we should send in our offering for that week or give it the next week/Sunday along with the offering for that current week (2 offerings) as God has commanded. We DO NOT keep what God has blessed us with for that week without returning a generous portion to Him for that would be “stealing from God” or “robbing God” which still has severe consequences (1 John 3:4; Jas. 4:17; Rom. 6:23; Rev. 20:14). Giving to God as He has prospered us is commanded every single week (1 Cor. 16:1-2). If for some reason we are unable to assemble with the brethren, we are still obligated to give back to God for that week.