Cultus Mountain is a peak east of Mt. Vernon. The land surrounding the mountain is a patchwork of public and private property. Some parcels are owned by Washington State Department of Natural Resources, but others--most notably those near the summit--are privately owned. Without a detailed map of land ownership it is tough to avoid trespassing here.
Also the routes through this forest are primarily on logging roads, so your outing could be interrupted by an active logging operation.