What is salvation?Salvation is eternal beatitude with the Lord. By that I mean, perpetual communion and happiness with the Lord, forever in his presence, in ecstasy before the very face of God. Since we only receive this upon entrance into heaven, salvation is rightly understood as something we do not ultimately experience until we die. However, we receive a taste of this salvation on earth through the grace of the Lord, which compels us to do good and to avoid evil, and that cleanses us of all sin and renews our relationship with the Lord.
If someone has doubts about whether they really believe or not and isn't sure how much they really believe or want to believe, where is the line drawn between saved and experiencing doubts and losing salvation because of a lack of faith and meaningless actions?Well, first of all, salvation (understood as eternal beatitude with the Lord) is not something you "lose" because, as Catholics, we believe that you never ultimately gain it in this life to begin with. Salvation is not something you receive once and for all time in this life. Instead, salvation is a process that culimates with entrance into heaven.
However, what we do "gain" and "lose" in this life is the presence of sanctifying grace, or the "state of grace." As the catechism says:
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace.Now, as for your question: can a lack of faith or the existence of doubt about what we believe as Christians cause the privation, or the lack of sanctifying grace? It depends on the nature of the disbelief.
As human beings, we are creatures that naturally question and doubt, especially about those things we cannot see. So, a certain element of disbelief is to be expected. However, when such doubts arise, we must work to overcome them. We can do this by asking questions and using our ability to reason, which God gave us so as to know him better. With things that are mysteries (like the Trinity) which, although they are logically consistent, are beyond our ability to fully understand, we must pray that God will grant us the wisdom to understand these things as much as possible and the faith and courage to believe the rest solely because it is True.
As long as we are working to overcome our doubts and our lack of faith, we cannot be said to be in a state of sin. We are not perfect creatures, and the Lord knows this. However, some people take their doubt and their inability to understand as reason to reject the Truths of the Church altogether. They willfully persist in doubt, and disbelief, and faithlessness. These individuals sin gravely and, although it is not up to us to judge the state of any man's soul, we can rightly say that such a person is in serious danger of eternal damnation, were he to die in this state of faithlessness.
Moral of the story: if you struggle with doubt and disbelief, don't give up. Pray, study, pray, pray, repeat.
Mark 9:24 "I believe, help my unbelief!!"He will help you
So really it's like as far as believing and stuff is concerned, someone is ok as long as he doesn't walk away from God?Well, it may be more correct to state it positively: someone is ok as long as he is trying to overcome his doubt.
What if someone who has doubts doesn't want to deal with them at the time they come up?It depends on why he doesn't want to deal with them (I know, nothing is ever black and white is it!). As the CCC states (see no. 1860), the promptings of feelings and passions, as well as external pressures and pathological disorders can diminish or even remove the imputability of a grave offense. However, I wouldn't depend on this for too long. Our Lord is merciful, but he is also just. If we spend too much time neglecting these doubts, then that could be seen as willful rejection of the Lord. It is good for us to always be working towards greater knowledge of and communion with Him.
Is it better to try to find out if you have doubts or is it better to just go on and let them come up if they will?Well, the only difference really is between doubting now and doubting later. I say go ahead and search your mind and see what you don't understand. The sooner you do that, the sooner you can begin to equip yourself with the knowledge that will be helpful both to you and to other people you meet who have the same doubts that you had.
I hope that helps.