David Wilkerson was happy in his Pentecostal pastorship of Philipsburg, N.J. when he first felt the call to help teen-agers in New York City. It was the story of The Michael Farmer murder and the faces of the defendants in Life magazine that set him on a strange and marvelous journey. With his congregation's blessing and financial support, he went to New York, and in trying to see the Judge was bundled off to jail. This at first seemed to be an embarrassing setback, but proved to be his making -- for the teen-agers he met on the streets, the gang members, felt he was one with them for his brush with the law. This incident sets the tenor of the story -- for each stumbling block proves to be an open sesame, each hurdle is whittled down to size for a public- praying, mission-minded preacher. Blocked from helping the defendants in the Farmer case, the Reverend Mr. Wilkerson extended his ministry and reaped converts in the toughest gangs. Then came the Teen Challenge Center in Brooklyn where sick teen- agers could find home, where the fight against heroin was won through the Baptism of the Holy Spirit with the help of such people as Nicky, a reformed gang member now of the ministry, and Linda, a brave volunteer from the midwest. Told in personal terms, this is a smooth-reading story of faith rewarded -- perhaps more constantly than those on more formal terms with God can accept.