6:36-38 What fundamental spiritual law is Jesus teaching here?

Jesus is teaching us here some very important principles of the law of sowing and reaping which apply to every aspect of our Christian walk - the giving of ourselves, our finances, and our time to others.

Firstly though, it should be noted here that while Christians should never give in order to receive, the law of sowing and reaping is a principle of life already established by God, and Christians have no say in the matter - giving and receiving go together (cp Pr 11:24-25; 19:17; 22:8-9; Ec 11:1-6; 2Cor 9:6-10; Ga 6:9-10);

It applies to our financial support of the ministry (cp 1Cor 9:4-11; Ga 6:6-7; Php 4:10-29; 1Ti 5:17-18); it applies to our moral behaviour (cp Pr 6:32-35; 11:18, 31; Hos 10:12-13; Ro 1:18-27; Ga 6:7-8),

And it applies to our Christian service (cp Psa 126:5-6; Pr 11:30; Dan 12:3). Our every attitude and action toward others will eventually reflect back on us. Just as surely as everything in nature reproduces after its own kind, harvests being as sure as sowings, so everyone of us will reap what we sow in this life and be solely responsible for our destiny in eternity (cp Gen 8:22; Pr 21:13; Mt 5:7; 6:14-15; 7:1-2; Lk 6:31, 36-37; Ga 6:7-8; Jas 2:13).