I don't understand today's Gospel reading.

It's flesh / spirit dichotomy.

Supposing you chose one building to protect in your city: the church or the bank. One might choose the bank because without it, everyone goes poor, and they don't think the spirit matters. One should choose the church, because if people are spiritually poor, then their prosperity doesn't matter because their characters rot since they are severed from spirituality. "It is the spirit that gives life; the flesh is useless" (see John 6).

You see the line later in the chapter (and Catholic gospel readings are shortened to a sample of a chapter rather than the whole thing, which may deny a little thematic meaning or context): in vain do they worship me, teaching human precepts as doctrine.

God is a spiritual God... Jesus came to emphasize that spirit > flesh. Naturally, his opposites, the scribes and the pharisees value flesh > spirit, and teach their followers to do the same, which rots their characters.

Looking at the ten commandments, the first tablet teaches spiritual law while the second teaches law of the flesh. The first tablet teaches "Love your God with all your heart, might, mind, soul" while the other tablet teaches you to "Love your neighbour as yourself". The rich man, for example, followed all of the second tablet laws but did not acknowledge the first: loving God with ALL your being (you cannot serve both God and wealth).

God always emphasized his spiritual aspects and the Pharisees tended to emphasize his material aspects hence the comparison between the gold and what makes the gold sacred (God, the essence beyond the gold): to the Pharisees: following God was to some earthly benefit... you'd get rich and make a lot of friends.

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