Mark writes in this chapter about the increasing conflict between Jesus and the various religious authorities. First the Pharisees and the Herodians (Pharisees were the ultra orthodox religious folks -- the Herodians were a political party working in collaboration with the Roman government) who ask a question that is part theological and part political. Is it lawful to pay taxes. If God is the sovereign ruler of the Israel then supporting any foreign government would be a form of idolatry. The political question is trickier: When one rose to power in the ancient world the first act was to mint coins with one's name and image on it. This displayed ownership of the currency of the realm. The coins, in fact, belonged to the ruler and were used by everyone to trade and buy and sell. Is it lawful to pay taxes? Since the image and likeness of Caesar is on the coin it belongs to him -- return it to him. the bigger question is: What is the image and likeness that should be returned to God? (see Genesis 1:26)
Next up are the Sadducees. Dr. Jerry Horner, New Testament professor at college, used to tell us: they do not believe in the resurrection that is why they are "sad - u - see" (makes it easy to remember them). The question of the seven brothers for one bride, though rooted in the Old Testament law, is ridiculous and Jesus treats it as such. Notice his description of our post resurrection selves.
The question about the great commandment -- there were hundreds of commandments in the Old Testament and thousands more that made up the "oral Torah". If 100 Rabbis of Jesus time were asked this question nearly all of them would have given the same answer Jesus did. From the Old Testament, the heart of the matter is the unity of God and our call to love God with heart, soul, mind and strength (Deuteronomy 6:4-5). The second is to love your neighbor as you love yourself. Rick Warren once wrote that a great commitment to the great commandment and the great commission (Matthew 28:16-20) leads to a great church. A church that is committed to loving God and our neighbor and going out to make disciples for Jesus will make a huge impact for the gospel of Jesus Christ in this world!