He is accused of knowing about the attack
The 30-year-old, named as Farid K, appeared before a judge in Paris as Muslims across France displayed their solidarity with the Catholic community in light of the gruesome killing.
Farid K, from Nancy, is the cousin of Abdelmalik Petitjean, who was shot dead by police after he and another man, Adel Kermiche, slit the throat of Father Jacques Hamel, 86, in his church in Saint-Etienne-du-Rouvray.
He is accused of knowing that his cousin would imminently commit a violent attack - though not the place or the date.
Petitjean's stepfather has said that the pair had spent time together and at times lived together. Another man, a Syrian refugee, was released by police, while a minor initially detained in connection with the attack will not be prosecuted over the incident, though may face separate charges in relation to materials found on his computer.
At Rouen cathedral, near the church where the victim was killed on Tuesday last week, more than 100 Muslims gathered to join in prayer with Catholics.
During the service, which focused on the memory of Father Jacques, Rouen Archbishop Dominique Lebrun expressed his gratitude for their gesture.
He said: "I thank you in the name of all Christians. In this way you are affirming that you reject death and violence in the name of God."
He added he was grateful for their "courageous act", as some within the Muslim community believe it is not acceptable to enter a church.
The head of the French Muslim council had proclaimed "we are all Catholics in France" in the wake of the attack, and called on the community to show their "solidarity and compassion".
In Lyon, a "brotherhood march" followed the religious ceremonies, where hundreds marched in silence carrying banners that read, "This is not a religious war" and "We are all brothers and sisters".
Prime Minister Manuel Valls has called for a new "pact" with the Muslim community in France, after renewed fears of possible religious tensions.
He said: "Islam has found its place in France ... contrary to the repeated attacks of populists on the right and far-right.
"This intolerable rejection of Islam and Muslims... must be combated - and it is - with the greatest strength."
The French government has come under pressure over perceived security lapses, following the Bastille Day attack in Nice, when a jihadist drove a truck through crowds, killing 84 people.
Mr Valls was booed as he attended a tribute to the victims, and faces further questions after it emerged that the two Normandy attackers had been on the radar of the intelligence services and had previously tried to enter Syria.