Is complete trust in God necessary for salvation, according to MSLN?
I'm not sure I can come up with a categorical "yes", but I think I can get close to a "yes".
As I study the Bible, I may come up with a stronger case, but I can at least say that to the extent that a person believes in the Bible through MSLN, we can see Jesus in the Sermon on the Mount teaching that a person should not be anxious and should ask God for what they need (whether they objectively or only psychologically need it, I guess). Not being anxious is a form of trust, and asking for what you need is a form of trust. Jesus ends the Sermon on the Mount with these words (Matthew 7:)
21 "Not everyone who says to me, 'Lord, Lord,' will enter into the Kingdom of Heaven, but he who does the will of my Father who is in heaven. 22 Many will tell me in that day, 'Lord, Lord, didn't we prophesy in your name, in your name cast out demons, and in your name do many mighty works?' 23 Then I will tell them, 'I never knew you. Depart from me, you who work iniquity.'
24 "Everyone therefore who hears these words of mine and does them, I will liken him to a wise man who built his house on a rock. 25 The rain came down, the floods came, and the winds blew and beat on that house; and it didn't fall, for it was founded on the rock. 26 Everyone who hears these words of mine and doesn't do them will be like a foolish man who built his house on the sand. 27 The rain came down, the floods came, and the winds blew and beat on that house; and it fell -- and its fall was great."
I'm not sure all of the Sermon on the Mount's commands can be seen as absolute moral commands, or if some of them can be seen as practical advice that is upstream of obeying moral commands. However, some of the commands, like not being angry, and forgiving, do have language that suggests they are moral commands, related to whether you will be saved, and so it is with anxiety (Matthew 6:)
Vss. 24 - 25 suggest that to be anxious is to serve Mammon -- maybe not an airtight inference, but I think strong enough to make us concerned that if we never stop being anxious, we will be kept out of heaven. We would be trusting uncertain things (being anxious) rather than trusting the certain person.
24 "No one can serve two masters, for either he will hate the one and love the other, or else he will be devoted to one and despise the other. You can't serve both God and Mammon. 25 Therefore I tell you, don't be anxious for your life: what you will eat, or what you will drink; nor yet for your body, what you will wear. Isn't life more than food, and the body more than clothing? 26 See the birds of the sky, that they don't sow, neither do they reap, nor gather into barns. Your heavenly Father feeds them. Aren't you of much more value than they?
27 "Which of you by being anxious, can add one moment to his lifespan? 28 Why are you anxious about clothing? Consider the lilies of the field, how they grow. They don't toil, neither do they spin, 29 yet I tell you that even Solomon in all his glory was not dressed like one of these. 30 But if God so clothes the grass of the field, which today exists and tomorrow is thrown into the oven, won't he much more clothe you, you of little faith?
31 "Therefore don't be anxious, saying, 'What will we eat?', 'What will we drink?' or, 'With what will we be clothed?' 32 For the Gentiles seek after all these things; for your heavenly Father knows that you need all these things. 33 But seek first God's Kingdom and his righteousness; and all these things will be given to you as well. 34 Therefore don't be anxious for tomorrow, for tomorrow will be anxious for itself. Each day's own evil is sufficient.
That may be enough to convince some people (it would be enough for me), but there may be other Bible passages that strengthen the case.
Also, for those who do not trust in the Bible as much, but only in the natural theological part of MSLN, what can be said philosophically?
It does seem odd to think of someone who refuses to cease being anxious entering God's rest. It may be "physically" (psychologically?) impossible for a personal being to be in communion with rest if they are anxious. It sounds true by definition. It is as though we condemn ourselves to something like hell if we do not cease to be anxious. Not being anxious is a big part of trusting.
Can God rest if we don't rest? According to the metaphysical organism argument, God experiences exactly what we experience, so, no. That's a fairly airtight reason to think that anxiety must cease, and that if our will is what keeps it going, it must change or we must be destroyed.
Is it possible to sustainably exist in an environment forever if you don't fully trust it? I can think of multiple cases where I was in a social environment that wasn't feeding me (or wasn't feeding me "good food"), and I sort of died inside and had to escape. I needed my "food" from somewhere, and had to flee the sterility or the sickness to find it. I didn't trust my environment, because it wasn't offering me what I needed, or what I hungered for. A person in God's presence who did not fully trust him, was not fully and on all levels receptive to him, perhaps hungry for him enough to keep up with his richness, might find him to be "bad food", or might want to "eat" something outside of God (some kind of sin). Then that person would become desperate and start acting out, but would have nowhere to escape to without disturbing God's rest.
You can resolve to be friends with God forever with your will (with you, the real you), but if your "body" (or whatever part of your psychology is really where the "organ of trust" is located) can't stomach God, you will be tempted over and over by your indigestion of him to turn against him. If you model temptation (as I think maybe I would, although I haven't thought about this as much as I might) as "roll an N-sided die and if you roll a 1 you give in", then to be tempted by any finite-sided "temptation die" over the course of everlasting time, at some point you will roll a 1 and turn against God. You might be able to turn back to God after giving in (or maybe not), but in any case it would disturb God's rest. Therefore it is best for us to learn to trust God fully before we enter his rest.
(Supposing that your conscious love for God is unshakeable, so that you can roll the die and never roll a 1, every time you're tempted by your "body" to reject God, you will experience conscious stress, which will disturb God's rest.)
Another dimension of trust connects to loyalty. If you follow someone, you trust them on some level (true even on social media). To put full and unwavering allegiance in someone is to trust them, to trust that they satisfy whatever criteria obtain for them to be the thing in which you should put full and unwavering allegiance. The phrase "give your heart" to someone sounds like we are putting something valuable in the care of someone else, which is a form of trust.
This may not be a complete list of reasons why we must come to fully trust God, but it's probably sufficient for now.
When I started this post, I felt like I couldn't say for sure ("categorically") that complete trust of God was necessary for salvation. But I think it's necessary for salvation, for reasons given above.
(I don't always stop to acknowledge my intellectual sources, mainly for having so few of them, not wanting to repeat the same names over and over, but I will say that a lot of my concept of trust comes from Joseph Godfrey's Trust of People, Words, and God.)