I see what you're saying here, but let me put it this way...given a few scriptures.
Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is--his good, pleasing and perfect will. -Romans 12:2
Now, I see how people can make this say what they want it to say, but I believe it tells us not to 'follow the crowd'. Most often, look at what the world says we should think, feel, do...and you'll find it in direct opposition to the scriptural (literal and non...not strictly either) understanding of what God would have us think, feel, do.
One of the ways this is instilled is through doubt, like you suggest, BUT...I have also found that within 'religious' circles...this idea of doubt being weakness. This is FALSE! In fact, if you don't ever call things into question, I would argue, that you never understood them, don't now, and won't until you ask, seek, try, analyse, test, and discover what something truly is or who people truly are.
Now, that said...here are a few more. Just a few, as there are many, and they speak toward this idea of evaluating things in life...including people.
Stay away from a foolish man, for you will not find knowledge on his lips. (Proverbs 14:7)
Wisdom will save you from the ways of wicked men, from men whose words are perverse, who leave the straight paths to walk in dark ways, who delight in doing wrong and rejoice in the perverseness of evil, whose paths are crooked and who are devious in their ways. (Proverbs 2:12-15)
But the wisdom that comes from heaven is first of all pure; then peace-loving, considerate, submissive, full of mercy and good fruit, impartial and sincere. (James 3:17)
Matthew 15: 1 Then some Pharisees and teachers of the law came to Jesus from Jerusalem and asked, 2 “Why do your disciples break the tradition of the elders? They don’t wash their hands before they eat!”
3 Jesus replied, “And why do you break the command of God for the sake of your tradition? 4 For God said, ‘Honor your father and mother’[a] and ‘Anyone who curses their father or mother is to be put to death.’ 5 But you say that if anyone declares that what might have been used to help their father or mother is ‘devoted to God,’ 6 they are not to ‘honor their father or mother’ with it. Thus you nullify the word of God for the sake of your tradition. 7 You hypocrites! Isaiah was right when he prophesied about you:
8 “‘These people honor me with their lips,
but their hearts are far from me.
9 They worship me in vain;
their teachings are merely human rules. ’[c] ”
10 Jesus called the crowd to him and said, “Listen and understand. 11 What goes into someone’s mouth does not defile them, but what comes out of their mouth, that is what defiles them.”
12 Then the disciples came to him and asked, “Do you know that the Pharisees were offended when they heard this?”
13 He replied, “Every plant that my heavenly Father has not planted will be pulled up by the roots. 14 Leave them; they are blind guides.[d] If the blind lead the blind, both will fall into a pit.”
15 Peter said, “Explain the parable to us.”
16 “Are you still so dull?” Jesus asked them. 17 “Don’t you see that whatever enters the mouth goes into the stomach and then out of the body? 18 But the things that come out of a person’s mouth come from the heart, and these defile them. 19 For out of the heart come evil thoughts—murder, adultery, sexual immorality, theft, false testimony, slander. 20 These are what defile a person; but eating with unwashed hands does not defile them.”
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Now, that said...there are people who are good for you to be around, and others who will lead you astray.
As far as trust issues and issue of this topic, one has to test, doubt, question, and understand something or someone before they can trust or not trust it.
What we've seen...(sort of the royal 'we' here)...is that people hurt people, and this causes lack of trust in humanity. It's also, statistically, not an isolated incident, but appears to be strongly correlated. Being hurt = trust issues. Now, though not always the case, it is largely this case the majority of the time.
So, from there, we're not saying that a person 'doesn't' let people in, but that they have difficulty doing so, because the most common, recurring result is being (not minorly, but seriously) hurt by doing so.
Christ calls us to be in relationship with Himself and those around us, but this issue of trust only ever appears in scripture when referencing God. Therefore; our trust of others largely becomes a matter of personal judgement.
I hear what you're saying, and respect that, but others are equally persuaded in doing what they understand as 'wise' or 'God's Will' that counteracts (our) your understand.
Many like to say that they are right, and others are wrong. What happens when both people are truly close to Christ, but arrive at different places on the same issue. One thing you may find with time, is that, there are things greater than the 'boxed' way we understand them. Perhaps both could be right, even in seemingly opposing positions.
So, like I said before, we're talking about what we believe in the case of trust or lack thereof within romantic relationships as a Christian. We trust God. We try to trust others, but it's difficult, complicated, and very messy/personal.