His parents, burgher Pietari Juusten and his wife Anna, owned their townhouse near the Blackfriars' monastery at Viborg.
The bishop ordained Paulus as a priest in 1540, before he turned 24 that was the regularly required age for ordination.
Juusten was acting headmaster of the Viborg school 1541–1543, after which the bishop sent him to Germany, to study in several universities.
The king sent bishop Juusten as ambassador to Russia in 1569, where tsar Ivan IV the Terrible kept him as a prisoner for over two years.
[1] Juusten authored the Finnish-language catechism (1574), the tale of his Moscow travel, a Mass book (1575) and the Chronicle of Finnish bishops, Suomen piispainkronikka, Chronicon episcoporum finlandensium.