Solidarity is not just a common generic noun, such as we find in a dictionary. It's a fundamental principle of Catholic social teaching and figures prominently in some important Papal documents. It appears in the Catechism in several places, and can easily be traced to Old Testament injunctions. The greatest example of solidarity is Jesus' solidarity with humanity. It's why he submitted to death on the cross - a death designed for criminals - though he himself was sinless.

Solidarity is idealized in Acts 2:44-47 and the following...

We can also find the ideal in the parable of the good Samaritan, in Luke 10:30-37. Indeed, we already find the roots of this teaching in Leviticus 19:13-18...

With that as background, we can more fully grasp the significance of the definition we find in the Oxford English Dictionary...

For Christians, the unity of agreement ... among individuals with a common interest is about holy communion — union with God in the Body of Christ. It's what motivates the corporal and spiritual works of mercy and the desire to build a kingdom of God. What it means for people will tend to differ based largely on whether they are motivated by a desire for security, a thirst for power or by sacrificial love. It will also tend to differ based on the source of their inspiration. As Jesus put it, "By their fruits you will know them[fn]Cf. Matthew 7:15-20.[/fn]."

One of the most dramatic examples of misplaced solidarity is the story of Saul of Tarsus, whose solidarity with the teaching of the Pharisees led him to persecute the Christian upstarts. I think one can see that he was as aflame for the Mosaic covenant as Judas Maccabeus[fn]See 1 Maccabees 2:66-68 and following chapters.[/fn]. Jesus knew of Saul's passion for the Jewish faith he was trained in, and recognized he was acting out of ignorance....

But Saul, still breathing threats and murder against the disciples of the Lord, went to the high priest and asked him for letters to the synagogues at Damascus, so that if he found any belonging to the Way, men or women, he might bring them bound to Jerusalem.

Now as he journeyed he approached Damascus, and suddenly a light from heaven flashed about him. And he fell to the ground and heard a voice saying to him, "Saul, Saul, why do you persecute me?" And he said, "Who are you, Lord?" And he said, "I am Jesus, whom you are persecuting; but rise and enter the city, and you will be told what you are to do."

The men who were traveling with him stood speechless, hearing the voice but seeing no one. Saul arose from the ground; and when his eyes were opened, he could see nothing; so they led him by the hand and brought him into Damascus. And for three days he was without sight, and neither ate nor drank[fn]Acts 9:1-9. Saul was a student of Gamaliel. See Acts 5:34-39 and Acts 22:3.[/fn].

While Saul's solidarity with his teacher and his party - the party of the Pharisees and their traditions of interpretation of scripture - led him on a campaign to destroy Christianity, the new rival, his conversion changed not just his loyalty, but his entire approach to advocacy. He was no longer a man of violence. He was no longer emulating the zeal of Judas Maccabeus. As a disciple of Christ he became a man of peace, using persuasion and teaching to build up the people of "The Way". His whole manner of life changed. His sense of solidarity was no longer limited to a religious party. He became a person in love with humanity as a whole and he was driven to save it. His exercise of that new orientation, however, underwent development and growth. He became more and more like Christ, himself[fn]Cf. Galatians 2:20[/fn].

That brings us to the unique solidarity of Jesus Christ with fallen humanity. Consider the following:

Jesus' early life was lived in very humble circumstances.

Jesus was born in very humble circumstances, in a cave used as a stable. When he became a teenager, he apprenticed to his stepfather, Joseph, as a carpenter/stone mason (a techton in Greek), an occupation he practiced in early adulthood until he began his ministry.

Baptism in the Jordan [Matthew 3:13-17]...

Then Jesus came from Galilee to the Jordan to John, to be baptized by him. John would have prevented him, saying, "I need to be baptized by you, and do you come to me?" But Jesus answered him, "Let it be so now; for thus it is fitting for us to fulfill all righteousness[fn]"All righteousness" clearly involved receiving John's purification rite so that the sinless Jesus would identify - be in solidarity with - sinful humanity.[/fn]." Then he consented. And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, "This is my beloved Son, with whom I am well pleased[fn]God planned this event to take place[/fn]."

The temptation in the wilderness [Matthew 4:1-11...

Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. And he fasted forty days and forty nights, and afterward he was hungry.

And the tempter came and said to him, "If you are the Son of God, command these stones to become loaves of bread." But he answered, "It is written, `Man shall not live by bread alone, but by every word that proceeds from the mouth of God[fn]Jesus' solidarity with sinful humanity entailed suffering as they did in their ordinary circumstances.[/fn].'"

Then the devil took him to the holy city, and set him on the pinnacle of the temple, and said to him, "If you are the Son of God, throw yourself down; for it is written, `He will give his angels charge of you,' and `On their hands they will bear you up, lest you strike your foot against a stone.'" Jesus said to him, "Again it is written, `You shall not tempt the Lord your God[fn]Jesus undertook to be subject to the vulnerabilities of humanity, an indication of his solidarity with vulnerable humanity.[/fn].'"

Again, the devil took him to a very high mountain, and showed him all the kingdoms of the world and the glory of them; and he said to him, "All these I will give you, if you will fall down and worship me." Then Jesus said to him, "Begone, Satan! for it is written, `You shall worship the Lord your God and him only shall you serve[fn]Jesus undertook to be born, and to live, in humble circumstances, and thus to live in solidarity with the poor and the ordinary.[/fn].'"

Then the devil left him, and behold, angels came and ministered to him.

He was hung upon a cross to die, the Roman punishment for a common criminal [Matthew 27:3-38...

And when they had crucified him, they divided his garments among them by casting lots; then they sat down and kept watch over him there. And over his head they put the charge against him, which read, "This is Jesus the King of the Jews." Then two robbers were crucified with him, one on the right and one on the left[fn]Jesus showed solidarity even with criminals, not to mention women trapped in a web of sin [See, for example, Luke 8:1-3, John 4:5-30 and John 8:1-10], people with all sorts of infirmities and society's outcasts.[/fn].

He even suffered what St. John of the Cross called a dark night.

And about the ninth hour Jesus cried with a loud voice, "Eli, Eli, la'ma sabach-tha'ni?" that is, "My God, my God, why hast thou forsaken me?"[fn]Matthew 27:46-50. This lament refers back to Psalm 22. It also contrasts with Jesus finding energy, refreshment and clarity of mission by spending the night in prayer, where it is believed he communed with God the Father, beholding the beatific vision.[/fn]

Jesus' solidarity with humanity is crucial to understanding why he had to die on the cross.

As Jesus hung upon the cross, he prayed to the Father to forgive those responsible,
   And Jesus said, "Father, forgive them; for they know not what they do." [Luke 23:34].
   See "Why did Jesus Christ have to die on the cross?".

In part 2 of this reflection, I will focus on connections of the principle of solidarity with other principles and values covered in the Compendium of the Social Doctrine of the Church.