You are correct that this does not refer only to the sin of apostasy. People may insert their own idea of what it means because they are hard-pressed to explain it.
The truth is, the sacrificial system never provided for any other than for a sin of ignorance. There was never a sacrifice for a willful sin.
Sins of Ignorance
Speak unto the children of Israel, saying, If a soul shall sin
through ignorance against any of the commandments of the LORD
concerning things which ought not to be done, and shall do against any
of them: . . . (Leviticus 4:2, KJV)
And if the whole congregation of Israel sin through ignorance, and
the thing be hid from the eyes of the assembly, and they have done
somewhat against any of the commandments of the LORD concerning things
which should not be done, and are guilty; . . . (Leviticus 4:13, KJV)
When a ruler hath sinned, and done somewhat through ignorance
against any of the commandments of the LORD his God concerning things
which should not be done, and is guilty; . . . (Leviticus 4:22, KJV)
And if any one of the common people sin through ignorance, while
he doeth somewhat against any of the commandments of the LORD
concerning things which ought not to be done, and be guilty; . . .
(Leviticus 4:27, KJV)
. . . And he shall take away all the fat thereof, as the fat of the
lamb is taken away from the sacrifice of the peace offerings; and the
priest shall burn them upon the altar, according to the offerings made
by fire unto the LORD: and the priest shall make an atonement for
his sin that he hath committed, and it shall be forgiven him.
(Leviticus 4:35, KJV)
The "sin through ignorance" is repeated again and again. God was making a clear statement that the sacrifices were only for sins of ignorance. There was never established any ritual, any ceremony, any fine or penance, nor any sacrifice or offering for a willful sin. Only sins of ignorance were included in the sacrificial type.
Jesus, the ultimate Sacrifice for our sins, reinforced the fact that he was dying for sins of ignorance.
Then said Jesus, Father, forgive them; for they know not what they do.
. . . (Luke 23:34, KJV)
Many see this as asking for forgiveness for the soldiers who were driving the nails. But this applied to every one of us.
Does this mean my sins will not be forgiven which I have sinned knowing that it was a sin?
This is the crucial question. How does God regard our sins which we have knowingly committed?
Even when we think we know that what we are doing is wrong, we are still quite ignorant. If we truly understood how much our sin would hurt God; how much it would injure our friends, family, and others; and how much it would hurt ourselves--we would not choose to do it.
The point at which we decide, however, to no longer listen to God's voice calling us to repentance, is the point where we are deliberately choosing our own wrong course. There is no forgiveness now, in part because none is asked. But furthermore, one of the most important lessons of this verse is that sins which are not forsaken will not be forgiven. When we know that something is a sin, yet we continue to commit that sin, it will eventually separate us from God.
Consider the Greek
For if we sin wilfully after that we have received the knowledge of
the truth, there remaineth no more sacrifice for sins, (Hebrews 10:26,
The word "sin" in the Greek (ἁμαρτανόντων/hamartanontōn) is a verb in the "present participle active" voice. This indicates that it is addressing an ongoing sin, not merely a one-time sin. The translators likely did not wish to add a word to the English translation that was not in the Greek, so they avoided inserting something like "continue to" before "sin." They could have said, perhaps, "are sinning," but that is not idiomatic in English--a bit like saying "I am having a master's degree" or "I am knowing about this." Some verbs in English seldom get used in the present participle form. But in Greek, this verb "sin" addressing a continuing action.
The text does not specify a particular category of sin, and may be seen to apply to any category of sin. However, it does imply an ongoing sin--continuing to sin after one has learned that it is a sin.
One sin, persistently cherished, will eventually neutralize all the power of the Gospel.
It is a fearful thing to knowingly and deliberately continue in a wrong course.