Following in his father's footsteps, Wallis then embarked on an itinerant preaching and teaching ministry, with a particular emphasis on revival, prayer, the work of the Holy Spirit, and the 'restoration' of the church. He had been deeply impacted by accounts of the Revival that took place on the Isle of Lewis in 1949 which he visited. His book "In the Day of Thy Power" (Christian Literature Crusade: 1956) was the fruit of this visit and his subsequent studies. He wrote eleven books on themes promoting the Christian life, and travelled widely (in particular to the US, Australia and New Zealand).