The man, the second to be held in connection with the killing of 85-year-old Jacques Hamel, was charged with "criminal association in connection with terrorism", after his arrest on Monday in the Toulouse area, the source said.
Just before the July 26th attack, the man had travelled to the Rouen region in northern France where he met Hamel's killers Adel Kermiche and Abdel Malik Petitjean, both 19 years old, according to a source close to the investigation.
The suspect, who is unemployed, claimed he was radicalised quickly and investigators believe he was mentally unstable, according to the source.
The attack on the church in the town of Saint-Etienne-du-Rouvray was claimed by the Islamic State group.
The two assailants were killed by police after slitting the priest's throat, injuring another person and taking five people hostage.
The grisly attack -- the first committed in the name of IS against a church in the West -- came less than two weeks after a Tunisian ploughed a 19-tonne truck into a crowd celebrating Bastille Day in the Riviera city of Nice, killing 85 people and wounding more than 300.
Previously unknown to French authorities, the suspect charged on Friday first contacted the priest's killers via the encrypted messaging app Telegram.
According to the source close to the investigation, the killers' numbers were both found on his phone.
The man told anti-terror police he had travelled on July 24th to Sainte-Etienne-du-Rouvray for what he described as a "course in religion" along with several other people, including the two killers, the source said.
The suspect told investigators he left the next day, claiming he had no indication of a plot to attack a church hours later.
However, they suspect he may have seen Kermiche's messages on Telegram, in which he described the modus operandi of the attack.
A cousin of Petitjean named as Farid K. was also remanded in custody on July 31st in connection with the attack.