피어선이 기독교 역사에 남긴 유산은 무엇보다도 복음주의 연합정신이었다. 이런 그의 유산의 기원은 1837년부터 1869년까지 존속했던 미국 장로교의 신학파(New School, 1837-1869)였다. 신학파의 부흥주의와 복음주의적 에큐메니즘 정신에 투철한 뉴욕 가정에서 태어나, 신학파 부흥의 중심지 뉴욕 주 북서부에서 학창시절을 보낸 데다, 신학파 장로교를 이끈 뉴욕 유니언신학교에서 공부한 피어선이 연합과 협력의 정신이 투철할 것은 이미 운명처럼 정해져 있었다고 해도 과언이 아니었다. 따라서 신학교 졸업 후 뉴욕과 디트로이트의 회중교회와 신학파 장로교회, 거의 초교파교회나 다름없는 필라델피아 베다니교회의 목회자로, 심지어는 런던에서 침례교인 찰스 스펄전의 메트로폴리탄 태버너클의 설교목사로 봉사하며 침례까지 받은 것은 피어선의 초교파 정신의 산물이었다. 피어선의 이름을 오늘날 가장 잘 대표하는 19세기 후반 선교운동은 처음부터 초교파 복음주의 부흥의 결과로 탄생했고, 그 구도 속에서 성장하고 발전했다. 피어선이 인생 후반기에 심취하게 된 세대주의 종말론과, 이 종말론에 근거한 선교운동, 그리고 신학적 근본주의 운동도 모두 현대주의의 신학적 개방주의에 저항하는 신앙적 보수주의자들의 초교파 연대였다. 19세기 중후반의 신학파 장로교, 2차대각성운동의 부흥주의, 여기서 파생된 노예해방, 금주운동, 성서공회 등의 자원선교운동, 학생자원자운동과 믿음선교도 모두 복음주의 에큐메니즘을 결집한 연합운동이었다. 마지막으로 선교100주년 런던대회(1888), 뉴욕 에큐메니컬 세계선교대회(1900), 에든버러 세계선교대회(1910)로 연결되는 선교운동도 모두 초교파적이고 복음주의적인 연합정신이 그 기저에 깔려 있었다. 피어선은 이 모든 연합운동에 적극적으로 참여하고, 자신의 목소리를 낼 뿐만 아니라, 글과 말, 그리고 그의 후계자들의 활약을 통해 유산을 남겼다.
The most significant legacy Arthur T. Pierson left in history was above all the ethos of evangelical ecumenism. This spirit of interdenominational cooperation arose from New School Presbyterianism which existed between 1837 and 1869. Pierson was born in New York City in 1837, when upstate New York as “the burnt-over district” was the centre of the New School Presbyterians. He was converted in a local Methodist Church in Tarrytown at the age of thirteen. It was an example of his life-long ecumenical and evangelical experiences that he crossed the border lines between denominations. Pierson attended Hamilton College in Clinton, New York, a town in the “burned-over district,” and Union Theological Seminary in New York City, the favoured training ground for New School Presbyterian ministers. Union Theological Seminary was founded by revivalistic Presbyterians as a place of ecumenical openness and partnership with other denominations in the Reformed tradition.” Henry Boynton Smith, a representative theologian in New School Presbyterianism, who had the most significant influence on Pierson, was professor at this school in Pierson’s seminary years. Smith helped Pierson combine his ecumenical tolerance of other evangelical denominations with his unwavering seriousness toward every word in the Bible. Two Presbyterian churches he ministered at, from 1863 to 1882, Waterford Church in New York and Fort Street Church in Detroit, were affiliated with New School Presbyterianism. Knox College in Galesburg, Illinois, which awarded an honorary Doctorate of Divinity to Pierson, was an affiliated institution of the New School related to the anti-slavery movement in the Midwest. On the other hand, Pierson linked himself to a new evangelical legacy as a central promoter of the post-bellum missionary movement, which was directly linked to Student Volunteer Movement beginning in 1888. This feature was revealed most obviously in his catchword, “the evangelization of the world in this generation.” In this phrase, which was first spoken at Moody’ s International Prophetic Conference in Northfield, MA in 1886 and soon became the slogan of the Student Volunteer Movement, he unfolded his premillennial mission theory. For Pierson, the hope of Christ’s Second Coming was the greatest motivation for world evangelization, and the motivating of the SVM watchword was its implicit belief that Christ would return once world evangelization was complete. Pierson’s influence based on evangelical interdenominational ecumenism was overwhelming. He was the lifetime supporter of the Evangelical Alliance; the editor of one of the most famous and interdenominational mission periodicals for twenty-four years; the writer of over fifty books including several on mission; the speaker at major revival and missionary conferences including two international missionary conferences in London in 1888 and New York in 1900; the founder of important evangelical institutions; the mentor of later great mission promoters such as John Mott, Robert Speer, Samuel Zwemer and Henry Frost; the minister of several large and important churches; and the administrative leader of the General Assembly of the PCUSA.