Like any time, art produced in 16th century Northern Europe reflected much of the common political and religious opinions. The biggest of these was a movement sweeping the Catholic Church in Europe. The Protestant Reformation encouraged art that spoke about a personal connection with God, a shedding of unnecessary sacraments, and a focus on the word of God.
The woodblock cut Allegory of Law and Grace is a good example of art preaching the difference between salvation from God and salvation from the leaders of the catholic church. On the left pannel, man seems to be persecuted by the Pope and others while on the right pannel, Jesus Christ is freely giving of salvation with almost no suffering involved. The medium, woodblock, allowed for this and many other messages to spread through print.
Another Protestant theme in art focused on the actual word of God. In the Four Apostles Durer painted the apostle John reading the word of god and standing in front of Peter, who represented the Pope in Rome.
Lastly, the Protestant Reformation argued that many of the seven sacraments of the Catholic Church were unneccessary and in many cases only separated man from God. Many ceremonies required the use of a priest or leader of the church and in the minds of the Protestants this was just a barrier between man and God's gift of salvation. One of the sacraments, the Eucerist, was only available through the hands of a priest in the Catholic Church. In The Last Supper Jesus presents the wine and bread as if to say that by its own nature, man can recieve God's gift of salvation.
The Art of 16th century Northern Europe helped spread the revolutionary Protestant ideas about God but these ideas were a small part of a growing humanist focus. Art in Northern Europe would soon become more secular and humanist, cutting many ties to any religious theme.