1) Believe in the Lord, and 2) keep His commandments.
1) does not only mean intellectual assent that God exists. It means belief in the correct doctrines concerning God - so, correct theology concerning Father, Son, and Holy Spirit, what was revealed about God and what He has done and does for us.
2) is inseperable from 1), because the God who gives us the commandments also gives us the means to fulfill them. You cannot say you believe in God without keeping His commandents. And indeed, it is impossible to fulfill the commandments without putting on Christ.
Heretics are those who proclaim false teachings regarding God or regarding what He has commanded, thus endangering the salvation of those who follow them.
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Find a priest who belongs to the Church, they are those who are given the authority to forgive or retain sins. Although it is only Christ working through them - they do not act on their own, but because they were given this mission and authority by Christ. It does not mean that God requires them for the sins of anybody to be forgiven, but, however, it does mean that you cannot be sure your sins have been forgiven unless a priest gives you absolution.