The path to Christian unity takes a willingness to acknowledge and share in the gifts other Christian communities have also received, Pope Francis said at an ecumenical Vespers service Friday.

To take the first step toward unity requires humble recognition of the fact that the blessings Catholics and other Christians have received do not belong to them by "right," but are a gift meant to be shared with others, he said Jan. 18.

"Then, we must acknowledge the value of the grace granted to other Christian communities," he continued. "As a result, we will want to partake of the gifts of others. A Christian people renewed and enriched by this exchange of gifts will be a people capable of journeying firmly and confidently on the path that leads to unity."

Held at Rome's Basilica of St. Paul Outside the Walls, the service marked the beginning of the Octave of Prayer for Christian Unity.

In his homily, Pope Francis reflected on the week's theme, "Seek to be truly just," inspired by the line from Deuteronomy which says: "Justice, justice alone shall you pursue, so that you may live and possess the land the Lord, your God, is giving you."

One of the grave injustices of today, he said, is the vast disparity in wealth which exists in many countries around the world.

"When society is no longer based on the principle of solidarity and the common good, we witness the scandal of people living in utter destitution amid skyscrapers, grand hotels and luxurious shopping centers," he said. "We have forgotten the wisdom of the Mosaic law: if wealth is not shared, society is divided."

He pointed out that in St. Paul's Letter to the Romans, the same idea is applied to the Christian community: "those who are strong must bear with the weak."

"Following Christ's example, we are to make every effort to build up those who are weak. Solidarity and shared responsibility must be the laws that govern the Christian family," Francis urged.

He also reminded Christians that it is a "grave sin to belittle or despise the gifts that the Lord has given our brothers and sisters, and to think that God somehow holds them in less esteem."

"When we entertain such thoughts, we allow the very grace we have received to become a source of pride, injustice and division. And how can we then enter the promised kingdom?" he asked.

"It is easy to forget the fundamental equality existing among us," he said, "that once we were all slaves to sin, that the Lord saved us in baptism and called us his children. It is easy to think that the spiritual grace granted us is our property, something to which we are due."

"The gifts we have received from God can also blind us to the gifts given to other Christians," he noted.

The Vespers was attended by representatives of various Churches and ecclesial communities, including the Eastern Orthodox, Oriental Orthodox, Anglicans, and students from the Ecumenical Institute of Bossey in Finland. Members of the Pontifical Council for Promoting Christian Unity were also present.