The tithe does not apply to Christians today, only to Old Testament Israel.
It is true that the tithe not the standard of Christian generosity today, for Jesus called us to give far more than 10 percent. In fact, the tithe was never the standard of Old Testament generosity, either. The tithe (as a mandatory gift equaling a tenth of one’s income) actually only applied to crops and animals from the promised land, not to all income in any time and place. Nor was 10 percent ever “the standard,” for there were three tithes (two every year, one every three years) and many other offerings and gifts that were also part of Jewish life. The idea of the tithe is still present in the New Testament (Matthew 23:23), but it is never explicitly applied to believers. Instead, almost all Christians are called to more extravagant freewill giving in response to the gospel of the Lord Jesus, who exhorted us to give 100 percent (Luke 21:1-4). The tithe may be a helpful guideline insofar as it reminds us to give proportionally to the Lord as he blesses us, since he owns it all anyway. But by New Testament standards, settling for 10 percent can be a recipe for condemnation from Jesus if we neglect true generosity, just as it was for the tithing Pharisees (Matthew 23:23). Biblical generosity is loving God with all we are, loving our neighbors as ourselves (Luke 10:25-37) and imitating the sacrificial example of Jesus (2 Corinthians 8:9; 1 John 3:16-18).
Christians are under grace, not under law, so the obligation to give does not apply to me.
The law/grace distinction in scripture is often misused, partly because the teaching is difficult to understand, but also because we sinners tend to seek out ways to dodge obedience. It is certainly true that Christians are under the principle of grace, not the principle of law. In other words, we attain righteousness not by behaving in the right way, but by trusting God for his salvation (Galatians 3:11). But this is not to say that we ought not to behave in the right way. The fact that we are saved by grace does not make adultery, for example, an acceptable practice. The law—God’s will for how we ought to live—is still a perfect standard and guide for us (Romans 7:12). But grace—God’s gift to us—makes obedience a possibility and a reality. Grace is not only forgiveness for past sins (although it certainly includes that), but also empowerment for future obedience. Grace does not mean that God does not expect us to give generously; it means that he will give us the strength to do it and the reward for doing it.
The Bible says that each man should give what he has decided in his own heart to give. So I am free to give as much or as little as I choose, even not at all.
It is true that the apostle Paul says, “Each man should give what he has decided in his heart to give, not reluctantly or under compulsion” (2 Corinthians 9:7). We might think, then, that the apostle is giving us liberty to be as selfish as we like. But curiously, this verse is situated in the middle of a passage urging Christians to give generously (vv 6-15). How can this be? Paul’s point is not to give license for selfishness, but to cultivate a love of generosity within his listeners. He does not want to “hold their hands” through the giving process. Rather, he wants them to feel free, creative and excited to give far beyond the ten percent benchmark. Because Jesus has accomplished our redemption and the age of the Spirit has come, Paul’s logic goes, appealing to the tithe laws is unnecessary. Yes, this is a sort of freedom unfamiliar to the Old Testament Jews. But it is a freedom to excel in good deeds, not a freedom to sin.