Sure, he could. He could do like he did with Thomas and physically reveal himself to every doubter, or blind every anti-Christian on a road, or have every rebellious believer swallowed by a fish, or do whatever outward sign that would remove all doubt from every unbeliever. But clearly God is pleased by faith and has chosen that to be the means through which salvation is applied. "Blessed are they who don't see and still believe." Even those of us who affirm "Contra-Causal" Freewill acknowledge that God has in the past and may in the future intervene with circumstances (signs and wonders or whatever) to ensure a specific outcome...(i.e. inspiration of scripture, crucifixion, etc)...but to suggest that EVERY circumstance, outcome, thought, evil intent, heinous crime and sinful act is likewise determined by God only serves to: (1) undermine the unique and divine attributes of those acts which SHOULD be attributed to God and (2) impugns God's Holy and sinless nature by making him appear culpable for originating sin.