Due to poor health, he remained in his union posts during World War I, supplementing his income by opening a bookshop.
[1] In 1919, the French Confederation of Christian Workers (CFTC) was established, and Tessier was appointed as its general secretary.
He worked with Jules Zirnheld to gain the blessing of the Vatican for their union activities, and refute accusations that they were Marxists, later being made a knight of the Order of St Gregory the Great.
He was a founder of Liberation-Nord, wrote for underground newspapers, and from 1943 served on the National Resistance Council, then on the Provisional Consultative Assembly.
In this role, he promoted Christian trade unionism internationally, and strongly opposed the growing support in the CFTC for secularisation.