The reading of Hurrell Froude's Remains and Frederick William Faber's Foreign Churches caused him to question that Protestantism alone represented the religion of the Apostles.

To set his doubts at rest, he visited Henry Edward Manning at Lavington, but felt so awed in the archdeacon's presence that he did not dare to enter into a controversy.

Subsequently, Manning urged Lockhart to accept John Henry Newman's invitation to stay with him at Littlemore and prepare for ordination in the Church of England.

After graduating Bachelor of Arts in 1842, he rejoined Newman at Littlemore and was assigned the task of translating a portion of Andrew of Fleury's History of the Church and of writing a life of Gilbert of Sempringham for the Oxford Series.

In this seclusion, his weakened faith in Anglicanism was shaken by the study of John Milner's End of Religious Controversy, given him by Grant, who had become a Catholic in 1841.

About a year later, however, his meeting with Father Aloysius Gentili of the newly formed Institute of Charity (Rosminians), at Ward's rooms, brought matters to a crisis.

He was the first of the Tractarians to become a Catholic, and his conversion greatly affected Newman, who shortly afterwards preached at Littlemore his last sermon as an Anglican, "The Parting of Friends".

After some years of successful preaching in various parts of England and Ireland, he was compelled, owing to ill health, to spend the winter of 1853 in Rome.