Sandius Jr. moved to Amsterdam and earned his living as an editor, translator and publisher, he became well-recognized among European intellectuals.
Sandius Jr. promoted the view that Arianism was the high point of the theology of the Early Church.
He remained on good terms with exiled Polish Socinians Andrzej Wiszowaty and Stanislaw Lubieniecki, while engaging in friendly polemics with them on the problem of the pre-existence of Christ which they denied, but Sandius, as an Arian, accepted.
Sandius Jr. also enjoyed close relationship with Benedictus Spinoza especially at the end of his life, when he became known as one of philosopher's most faithful followers.
Between 1676-1680 he defended the anonymous author of the Tractatus Theologico-Politicus in correspondence with the Catholic theologian Pierre Daniel Huet.