If or Since

We have come to share in Christ if we hold firmly till the end the confidence we had at first. Hebrews 3:14
Someone said that the word "if" should be translated "since" but as I checked other translations the word is "if" not "since". Now it would fit our doctrines much better if the word was "since" but it is a conditional phrase and we are told that we have come to share in Christ if... Does this sound like works-righteousness? However, we need to be reminded that we can only work out what God has worked in. Do we not like the word "if" because it challenges our doctrine?