A couple people told me I only have a head knowledge but not a heart knowledge and may not be saved which was very discouraging.
I would be discouraged, too. I started out in the charismatic movement. They tend to measure spirituality by their emotional response to the truths of God. That bothered me, even though I'm a very emotional person. I eventually learned what true spirituality, that confirmed the faith that produced it, looks like (in addition to other qualities):
"22 But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, 23 gentleness and self-control." (Galatians 5:22-23 NIV1984)
This is what we are to strive to produce in accordance with faith in Christ. It's not that there is no place for emotions in a relationship with Christ. It's just that emotions, along with outward religious duty, are a poor and deceitful measure of spirituality by themselves.
I know many Christ rejectors are sure they have the evidence of relationship with God and are, therefore, saved by a God they insist either doesn't exist or is different than the Bible portrays him to be, so the subject requires a little explanation, but fundamentally, it is the character that is growing up into the image of Christ that indicates true salvation.
For the Church, the nature of unstable, shifting, unpredictable, and ever-changing human emotions, that mostly depend on circumstances of body and life, alone is a very poor way to measure the grace of God in a person's life (again, not discounting emotions altogether). It is when you can see yourself being conformed to the image and character of God despite the discomforts of body and circumstances that you can be sure that the hope and confidence you have for salvation is real:
10 Therefore, my brothers and sisters, make every effort to confirm your calling and election. For if you do these things (the qualities of the Spirit he just listed--see text), you will never stumble, 11 and you will receive a rich welcome into the eternal kingdom of our Lord and Savior Jesus Christ." (2 Peter 1:10-11 NIV)
Also, I don't think anyone will miss out on salvation because he didn't have a huge amount of faith since Jesus said the faith of a mustard seed can move a mountain.
In regard to the faith to believe that God forgives sins through Christ Jesus I'd say this is not true at all. You have to believe this to the point of being born again. A partial faith about Christ won't do that.
This may very well be the fundamental problem in the church today. Lot's of people who know Jesus is the Christ, and who go through many of the outward religious motions, and emotions, of faith,
but who haven't truly come to a complete faith and trust in Him. The sin issue never quite gets settled for them with God and they languish outside of the grace of God, but by all worldly appearances they appear devout.
I agree with you that repentance must be sincere. I think as long as someone truly believes the truth that's been revealed and sincerely repents then he will be saved.
...The key being that what the person believes can be seen in what a person who genuinely believes becomes--a new creation that walks by the fruit of the Spirit (more and more).
Lot's of people acknowledge the truth. Few truly surrender their lives to it.
That surrender is how we can tell if we truly believe the gospel message. Most twist it, and rationalize it away, and conform it to their own liking and never enter into the transformed life that genuine faith in Christ brings. They will be sorely disappointed on the Day of Wrath.
The increase we bring to the seed and the talent of the Word of God that Christ will be looking for when he returns is the actual deeds of the Spirit, not just lip service or religious duty. You can't be saved on the Day of Wrath holding a fistful of seeds or a few talents of knowledge that you did nothing with. Christ is looking for increase. It's one thing to agree Jesus is the Christ and accept scripture as God given. It's quite another to live it. That's how you bring increase to your knowledge of God.
Live it, act on it, according to the fruit of the Spirit. Only genuine, justifying faith can do that.