The first apparent exception to this, Abraham’s giving a tithe of the spoils to Melchizedek (Gen. 14), is not sufficient to overturn the general principle, and surely is not sufficient to warrant perceiving the tithe of one’s income as ordinary and morally binding. First, Melchizedek was himself a priest, though he preceded the levitical priesthood. Second, he was a type of the greater priesthood of Christ (Heb. 5:6,16; 6:20; 7:1,10,11,15), so he does not disprove, but prove, the “priestly” character of tithing. Third, Abraham does not tithe to Melchizedek his income or regular property, but the spoils of warfare. Finally, Abraham’s offering of the tithe of the spoil to Melchizedek is freely given, not as a response to any God-given law. If the mere fact that Abraham did it made it lawful, then one might argue that Abraham’s polygamy is also lawful. That is, one may not make a narrative normative without some other consideration. All we know from the Abrahamic narrative is that it is permissible, albeit not mandatory, to tithe the spoils of warfare to any priest who is a type of the coming Christ; and since no such priests will appear after him, the permission becomes moot.
Melchisedek was a priest who was a type of the greater priest, Christ, so if tithing is "priestly" we may give tithes to Christ and His cause and kingdom.
We - normally - don't have the spoils of war to tithe, but as Christ's kingdom expands by the Holy War of mission and evangelism, there will be greater spoils
of that war to give and tithe from.
I'm only arguing for a freely given tithe, anyway, which Gordon doesn't contradict.
We know polygamy is wrong.
The second apparent exception is the tithe of Jacob (Gen. 28:22); yet it is evident here also that the tithe is freely offered, not as a response to any moral law or obligation. Indeed, Jacob’s tithe is the response to the extraordinary vision he has had, and demonstrates nothing regarding tithing being a general moral duty. If it is a duty to give a tenth, then it is also a duty to sleep on a rock, then make the rock a pillar and pour oil on it (Gen. 28:18). All of the difficulties with the preceding narrative are germane here also. A narrative is not normative without some other consideration. Further, Jacob’s vision of the ladder is as exceptional (i.e., not normal or normative) as Abraham’s meeting with Melchizedek. In neither case is any commandment issued; in neither case does anyone other than the principle of the narrative take the action; and in neither case was the tithe a perpetual act, but rather a one-time act. Would proponents of the tithe today propose that people tithe once and never again?
This passage may be the account of Jacob's conversion.
We know it is not a duty to sleep on a rock. It may be a necessity if we are in the wilderness. Thankfully, in God's grace, many of us have pillows.
Jacob's making the rock a pillar and pouring oil on it, teaches us to worship God appropriately and remember what He has done for our soul.
The considerations that make tithing stand out as emblematic and paradigmatic for us in these passages are
(a) Abraham and Jacobs' relation to us as covenant forefathers. We are in the New Covenant stage of the Abrahamic Covenant.
(b) Melchisedek's being typological of Christ, our priest.
(c) Melchisedek bringing forth bread and wine, as Christ does for His people.
(d) Jacob's conversion leading to him taking giving seriously.
(e) The continuation of tithing as a pattern, albeit mandatory for the minority of Israel/the Church, by God's people throughout the Old Testament.
(f) The command to give regularly in the New Testament.
(g) The absence of any guidance as to appropriate regular week to week giving apart from the tithe.
Otherwise you give what you feel spiritually is right from one week to the next, without any, at least general guidance, from Scripture, because you believe there is none.