Question:
In Hebrews 7:4-10 it shows that a tithe is 10%. We are told in 1 Corinthians 16:1, 2 to give money to the Lord each Sunday, as He has prospered us. What exactly does “as God hath prospered him” mean? Is 10% a guideline for us? Is it 10% of our gross income or income after taxes? Or should we throw the 10% idea out the window and give totally as we are led by the Spirit?

Answer:
First of all, the verses mentioned in Hebrews 7 are speaking of the priesthood of Christ, and they have no bearing on the subject of our giving to the Lord.
In addition to 1 Corinthians 16:1, 2 which Phil mentioned, I can think of two other portions of Scripture where this subject is found. They are 2 Corinthians 8:9-15 and chapter 9. In none of these Scriptures do we read of a tithe, or ten percent, being either an obligation or a guideline for believers today.
But in the Old Testament God obligated the nation of Israel to a tithe. Leviticus 27:30-33 is one among other Scriptures where we see this. The purpose of the tithe is seen in Numbers 18:21-24 and other similar Scriptures. I do not know of any Scripture that shows anyone else being obligated to the tithe.
Believers today, then, should be guided in their giving by the New Testament Scriptures mentioned above. If we speak only of the offerings taken in the assemblies, it should be done on the first day of the week (1 Corinthians 16:1, 2). It should be done out of a willing heart to further the work of the Lord.
Prayer in giving is very important. The Lord will guide us as to what amount we should give, if we ask. It is very important that all our financial obligations be satisfied before we give. It is also very important that we do not obligate ourselves so heavily that we have little or nothing left for the Lord. In today’s materialistic world this is easy to do. But, “he which soweth sparingly shall reap also sparingly” (2 Corinthians 9:6).
But God has prospered us, has He not? What does this mean? Aren’t there many of us who have more than we need? Couldn’t we return the overabundance to Him and then even give Him some more and really make a sacrifice? “He which soweth bountifully shall reap also bountifully” (2 Corinthians 9:6). (The sowing and reaping in both cases is spiritual and not material.)
In addition to the offerings in the assembly, we can make personal gifts to those in need and to other areas of the Lord’s work. We need much wisdom here. We need to be sure the personal need is real. We don’t want to support dissolute living. In other areas of the Lord’s work, we want to be sure we are supporting the LORD’S work. Would it be the LORD’S work if the truth of Scripture is not taught and obeyed? Perhaps even more wisdom is needed here.
In all of this I see no percentage guideline from God’s Word. What I see is giving as the Lord leads according to the means He has provided. And I also see the Lord providing the opportunity for us the givers to be His instruments of blessing. Have you ever thought that this might be the reason why the Lord has blessed us with the abundance that we have? Remember the sowing and the reaping in 2 Corinthians 9. And, finally, such giving glorifies God when it is done in such a way that only the recipient and the Lord know about our gifts.There is much more that could be said about giving, but perhaps this much will answer the question.