Our Gemara on Amud Aleph discusses the upper limit of charity, which is one fifth of earned income. The scriptural proof text comes from Yaakov (Bereishis 28:22), where he pledges to Hashem: “And of all that You shall give me, I will surely give a tenth of it [aser a’asrenu] to You”. The double use of the verb that means to donate one-tenth indicates that Yaakov, who issued this statement, was actually referring to two one-tenths, i.e., one-fifth.

What is the theological basis for this number, one fifth?  And furthermore, why doesn’t the verse just say “one fifth”, instead of implying one fifth by doubling one tenth?

Arvei Nachal (Vayetze 2:23) explains that one tenth is the number of the holy.  That is why a minyan can only be achieved by ten people. Thus, in order to bring holiness upon one’s possessions, it is necessary to offer one tenth to God.  

However, there are two stages and levels of achieving holiness; that is fear of God and love of God.  Fear of God must come first, and that is the first tithe.  The second tithe is the next stage of holiness which is love of God.  Fear must come first as enduring love can only take place within a context of good boundaries and commitment.  Indeed, our Gemara on Daf 51a tells us that a marriage where the woman does not either feel financially secure in the event of death of her husband or divorce, or if her husband can divorce her at whim with no consequence is considered a “be-iylas zenus” (see Tosafos), which is the Gemara’s idiom for a one-night stand.  Marriage and love without respect and responsibility is promiscuity, I say perhaps even masturbation.  So too, fear of God is a precondition to love of God.  This is also why the number is one fifth, and it is expressed in two stages. It emphasizes that it is not one fifth, but actually two sets of one tenth. That is, the bringing upon oneself two distinct stages of holiness: fear and love.