Many GPS devices try to bring you in from the south, off of Route 123 via Kings Highway (also King St, Millen Lake Rd).
The road between Route 123 and the Washington Church is a rough dirt road and should be avoided.
The only good way in is off of Route 31 in the center of Washington.
Several mapping services online locate the Washington Church quite accurately. Simply put "Washington Seventh-day Adventist Church, NH" as your destination.
Detailed directions from Route 31:
At the town center in Washington there is a white gazebo on the west side of Route 31.
Turn west onto Faxon Hill Road which is just south of the gazebo. Proceed out Faxon Hill Road approximately 2.1 miles.
Turn left onto King Street, which is a dirt road.
Proceed south on King Street for 0.3 miles and the Church will be on the left.
There is a parking lot on the left just prior to reaching the Church. Please park in the parking lot.
King Street, which is a dirt road, is not maintained for travel in the winter.
On October 7, 1974, this marker was unveiled designating the Washington New Hampshire Church a State Historic Site.
Read the full article here.