Six churches have been daubed with offensive and blasphemous graffiti.
Police believe someone with a grudge targeted the churches in Shoreham and Southwick.
Detectives said the messages were deeply offensive.
Five of the places of worship were defaced on Sunday night and the sixth attack was discovered on Wednesday.
The messages referring to God, Jesus and babies dying, were daubed in large letters designed to be clearly visible.
They were scrawled on church walls and noticeboards where clergy and members of the congregation could not miss them.
Those attacked were the Church of the Good Shepherd at Shoreham Beach; St Mary's in Southwick and St Nicholas, St Peter's, St Julian's and the Free Church in Shoreham.
Adur District Council sent in teams to help clean off the graffiti,which was scrawled in white paint, marker pen, chalk and crayon.
Almost all of it has been removed.
Detective Inspector Simon Harsley said: "The messages were extremely offensive and upsetting to those of us who are regular worshippers.
"They are too offensive to publish in a family newspaper and might cause a great deal of upset if repeated.
"All I will say is that they were anti-Christian and were about Jesus and babies dying.
"We believe it may be someone who has suffered a bereavement and is lashing out against the Church or somebody who has mental health issues.
"We don't believe it is connected with the conflict in Israel and Lebanon but we are keeping an open mind."
Father Sean Finnegan, of St Peter's Roman Catholic Church, said the church was shocked and saddened.
He said: "It's obviously not malicious but someone who's disturbed in some way.
"We feel sorry for them and pray for them. There's no anger, just sadness."
None of the churches are covered by CCTV and no reports have been received of anyone acting suspiciously.
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