because you don't take the context into consideration,
Hebrews 3:10-19 KJV
10) Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
11) So I sware in my wrath, They shall not enter into my rest.)
12) Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
13) But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
14) For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
Hebrews 3:14 LITV
For we have become partakers of Christ, if truly we hold the beginning of the assurance firm to the end;
Hebrews 3:14 YLT
for partakers we have become of the Christ, if the beginning of the confidence unto the end we may hold fast,
15) While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
16) For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
17) But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
18) And to whom sware he that they should not enter into his rest, but to them that believed not?
19) So we see that they could not enter in because of unbelief.
Greaved with what generation? The Israelites who came out of Egypt. Why? They did not believe. Warning: take heed, lest you have an evil heart of unbelief, departing from God. Don't be hardened by the lies sin tells you! Because we have become partakers of Christ, but that's only true if we don't lose our confidence. Just like the Israelites, who though they followed Moses out of Egypt, still died in the wilderness, because they did not believe.
They came out of Egypt, but they fell in the wilderness, because of their unbelief. So for us . . . you may come into a church, call yourself Christian, "follow Jesus", but if you, like them, are actually in unbelief, you, like them, will fall.
Much love!