Preach is defined by the Oxford dictionary as to
deliver a sermon or religious address to an assembled group of people, typically in church.
In Christianity preach usually means, to speak out the word of God to a congregation in a form of sermon.
Preaching can also be done publicly by proclaiming or teaching a religious message or belief.
Similar phrases to preach are: give a sermon, deliver a sermon, spread the gospel also known as evangelism or witnessing.
Preach does not always relates to religion, a belief or course of action which is earnestly advocated is a preach.