Westerbaen was born in The Hague, the son of a rope maker, and was initially schooled in the Staten-College, then as secretary and preacher of the Remonstrants of the Synod of Dort.
He showed, however, a domineering disposition and exercised a vehement polemic, as shown in his struggle with Cartesianism and the Remonstrants.
Later General Baptists such as John Griffith, Samuel Loveday, and Thomas Grantham defended a Reformed Arminian theology that reflected more the Arminianism of Arminius than that of the later Remonstrants or the English Arminianism of Arminian Puritans like John Goodwin or Anglican Arminians such as Jeremy Taylor and Henry Hammond.
After the general exile of Remonstrants from the Netherland he was at the Arminian colony of Friedrichstadt in Holstein.