A learned theologian, Heykamp is most remembered for summoning the conference that led to the Declaration of Utrecht.

Following the death of Henricus Loos, Archbishop of Utrecht, on 4 June 1873, Heykamp was consecrated Archbishop of Utrecht by Bishop Gaspardus Johannes Rinkel of Haarlem and Josef Hubert Reinkens of Bonn.

Heykamp immediately nominated and consecrated Cornelius Diependaal as Old Catholic bishop of Deventer, so that the three Old Catholic sees of Utrecht, Haarlem and Deventer were all filled for the first time since Bishop Lambertus de Jong’s death in 1867.

He also penned a protest against a petition by Roman Catholic bishops in the Netherlands to King William III, for the restoration of temporal power to the pope.

In 1880, he replied to an encyclical by Leo XIII that suggested that the civil marriage of Roman Catholics was not valid.