He was in Scotland in 1638, and signed the national covenant in presence of the General Assembly held at Glasgow in that year.
He asks Spang, minister of the Scots church at Campvere, to 'keep correspondence with that young man,’ and to urge him to 'use diligence' against the British sectaries in Holland, and to 'write against the anabaptists.'
After a short ministry at Delft he again became a military chaplain (apparently to the Scots brigade), and continued to officiate in that capacity till the Restoration.
The king Charles II, having restored episcopacy in Scotland, appointed Forbes, then chaplain to Andrew Rutherford, 1st Earl of Teviot, governor of Dunkirk, to the bishopric of Caithness, and with five others he was consecrated at the abbey church of Holyrood 7 May 1662 by the archbishops of St. Andrews and Glasgow and the bishop of Galloway.
James Kirkton, referring to his appointment to the bishopric, calls him 'the degenerate son of ane excellent father.'