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There is the external witness of God in the good provision of external things such as rains and fruitful seasons stated in today’s Scripture.
There is also the internal witness of God in the good provision of an internal conscience.
“… so that they are without excuse”. Romans 1:20.
Regarding the external witness, “For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness, because that which is known about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened. Professing to be wise, they became fools, and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures.” Romans 1:18-23.
Regarding the internal witness, “For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus.” Romans 2:14-16.
Reformed Doctrine Daily Devotions 