Romans 16:18 For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.

The Greek word for simple here is akakos.

Guileless (Without Guile):

lit., “without evil” (a negative kakos, “evil”), signifies “simple, guileless,” Rom 16:18, “simple,” of believers (perhaps == unsuspecting, or, rather, innocent, free from admixture of evil); in Hbr 7:26, RV, “guileless” (AV, “harmless”), the character of Christ (more lit., “free from evil”). Cp. Sept., Job 2:3, 8:20; Pro 1:4, 14:15.
See Harmless. (Source Vine’s Expository Dictionary)

Allow me to remaster the definition in a way that makes sense in light of the danger of deception which Paul speaks about in this opening verse.

Faithless acceptance.

It takes on the form of trusting men to do your thinking for you. It is easier to accept than it is to investigate. Good words and fair speech spoken by the Serpent are a perversion of the truth. It may sound good, but it isn’t good for you.

2 Timothy 2:15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.

That takes time, effort and sacrifice. It is easier to just buy the CD, DVD, or book and let someone else do the thinking for you. That is the relational equivalent of hiring someone to date your wife. It does nothing to improve your personal relationship with God.

Even this devotional, if used to replace real study time, is a hindrance to relationship if this is all you have the time for. None of us who provide you with encouragement of words would ever want this to replace your quiet time with God.

People who come to church each Sunday and snicker at those who only come at Christmas and Easter are no better off if the only sustenance they get is the Sunday Sermon.