This verse captures the essence of what a Christian absolutely must have faith in if he wants to conduct his life without falling into the same state of mind that Solomon did as shown in the book of Ecclesiastes. We, too, are subject to our own unstable convictions, opinions, and decisions.
In addition, we are subject to decisions and circumstances that others make and over which we have no control, yet which cause us to descend into a blue funk. We seem to be powerless over people making these decisions, so life seems unfair that such things should happen.
But we Christians cannot lose our perspective! Romans 8:28 is the right perspective for a Christian, a wonderfully encouraging and comforting promise. However, it does not automatically apply to everyone. Two conditions must be met.
First, we must respond to God's grace, to His gift, to His calling, to His gift of Christ, to His gift of the Holy Spirit, to His gift of revealing to us knowledge and understanding of what is happening. We must respond - that is, love God in return.
Second, we must be one of "the called according to His purpose," one of the elect. This does not apply to those who have merely received an invitation from God, because that summons goes out to many more than actually respond to it. Just as in advertising, the call, the invitation, may go out over radio, television, or through the newspaper to millions of people, but few respond as compared to the mass of invitees. The calling of God is similar: The invitation goes out to many, but few become part of the elect (Matthew 22:14).
If we meet these conditions, God is with us, and we can be encouraged and take comfort in that assurance.