Zhang Kai admitted to various crimes including disturbing social order in a broadcast on Thursday.
He has been helping defend Christians resisting government orders to remove crosses from buildings.
China says it guarantees religious freedom but there are concerns about a crackdown on Christian activities.
On Friday a pastor was jailed for refusing to remove a cross from his church's roof.
The authorities have justified the tearing down of crosses by saying they break planning rules.
Zhang Kai was arrested last year shortly before a planned meeting with the US envoy on religious freedoms.
In the broadcast, Mr Zhang said he had assisted the Christians because he wanted wealth and fame.
"I really regret doing these things, I feel very remorseful," he said.
"These things violated China's law and violated my personal integrity as a lawyer, and they harmed societal structure and national security."
China frequently airs confessions from high-profile suspects on television.
Condemning the broadcast, a US State Department spokesman said "such confessions are counter to the standards of a rule of law.
"We urge China to release Zhang and others detained for seeking to peacefully uphold the freedom of religion guaranteed in China's constitution."
The US says a purported confession from a prominent Chinese lawyer on state television runs counter to the rule of law.