Why were the Pharisees (Religious people) opposed to Jesus?

The religionists during the earth walk of Jesus were the Pharisees. Wanting to uphold all of God’s laws, they devised an intricate system of oral tradition to keep them from breaking the Mosaic law. But their reason for wanting to keep the Law was because they were afraid of the consequences of breaking it and would suffer the wrath of a angry God. If it wasn't for their self-righteousness they may have recognized the perfect obedience of Jesus and accepted the finished work of the cross and affirmed and followed Him. But, as demonstrated by the events recorded in Matthew 12:1–37, they were His most bitter opponents. Why was this?

The essential problem lay in their different understanding of the nature of God. For the religious Pharisees, as well as the religious people of today, God is primarily a God who makes rules to be obeyed. For the religious law and rule keepers of today, the bible is primarily a set of rules that must be kept at all costs. For Jesus, as well believers in His Gospel of Grace, God is primarily “gracious and merciful, slow to anger and abounding in steadfast love” (Ps. 145:8).

The ultimate cause of the Pharisees’ antagonism toward Jesus lay in His ignoring of their hundreds of elaborate rules that they had devised for interpreting the law of God. Not only did they devise these hundreds of man-made rules, but they had also elevated them above the level of Law, so that to break one of their rules was to violate the law of God itself. And yet these rules not only obscured the true intent of God’s law, but also, in some cases, actually violated it (see Mark 7:9–13). 

One of these violations that Pharisees upset with Jesus was the way He ignored their trivial and burdensome rule keeping. In Matthew 12 verses 1–8, the Pharisees objected to the disciples of Jesus plucking and eating heads of grain as they walked through the grain fields on a Sabbath. According to their oral tradition, plucking the heads of grain and eating them was work and was a violation of the Sabbath.

On that same Sabbath day, Jesus entered their synagogue where there was a man with a withered hand. Now, eager to again accuse Jesus of failing to uphold the Mosaic Law, they asked him, “Is it lawful to heal on the Sabbath?”. Before healing the man, Jesus answers their question by asking which of them, if his sheep falls into a pit on the Sabbath, would not lift it out. If, then, it is lawful to relieve the misery of a sheep on the Sabbath, how much more is it lawful to relieve the misery of a fellow human being who is more valuable than a sheep?

In both instances...that of the disciples eating the grain and of Jesus healing the man’s withered hand...the scriptural principle that Jesus applies is that God "desires mercy and not sacrifice”. 

Jesus also heals a demon-oppressed man who was blind and mute. Not having a Sabbath violation charge to bring against Jesus, the Pharisees now resorted to the slanderous charge that Jesus cast out demons by Beelzebub, the prince of demons (that is, Satan himself). Since Jesus cast out demons by the power of the Holy Spirit (v. 28), their slanderous charge was actually blasphemy against the Holy Spirit.

People of God, we need to be careful that we do not add our own man-made interpretations or rules to the Scriptures. Some beliefs that we hold dearly may be derived more from our religious culture or denominated denominations more than derived from Scripture, and we need to learn to discern the differences. Religious judgementalism and condemnation among religionists today is a result of the injection of man-made ideologies that are accepted as doctrines of Christ. But that is basically what the Pharisees were doing and what religion is continuing to do today.