Is that contempt? Yes they are, and that comes from Hosea 4:6.
Contempt? Lets explore Hosea 14:6
6 My people are destroyed for lack of knowledge.
Because you have rejected knowledge,
I also will reject you from being My priest.
Since you have forgotten the law of your God,
I also will forget your children.
Which lack of knowledge is destroying Israel? The 4th line of the verse tells you. They have forgotten the law of God. Since they have forgotten the law of God they are sinning against the Lord by not keeping His commandments. Keep reading the chapter and it tells you the controversy that God had with Israel.
This has nothing to do with birthdays.
And,
I don't know what we need for salvation, God knows that.
What we need for salvation is clearly depicted in His word. No guessing is necessary.
All I know is that we should make every attempt to make it through the narrow door.
We? Do we make it through the narrow door by our own actions and works? Can we effect salvation on our own?
Do you think you have some kind of license to sin because of your freedom in Christ? James, and Jude had plenty to refute that.
lol! Paul covers that in Romans 6. Read it carefully.
So how do you interpret Romans 12:2 then?
Lets look at it and brake it down.
2 And do not be conformed to this [
c]world, but be transformed by the renewing of your mind, so that you may [
d]prove what the will of God is, that which is good and [
e]acceptable and perfect.
And= adds to verse 1 and what our spirit of worship should be.
Do not be conformed= The verb συσχηματίζεσθε (suschematizo) means to
form according to a pattern or mold, to fashion alike, to conform to the same pattern outwardly. The meaning is to form or mold one’s behavior in accordance with a particular pattern or set of standards. The preposition "sun" in this compound verb denotes a personal assimilation to or conformity with the pattern indicated. It is interesting that the word includes "schema" where we derive our present word scheme. So here the word is used to
denote the practice of adopting for oneself a pattern or mold of life that is changeable and unstable rather than enduring. The verb tense is present but the voice is passive meaning that this is not an action that we do consciously but one that is exerted by an outside agency.
To this world: an age. Contextually here as "this age" characterized by a particular kind of existence. Paul further defines what he means is subsequent verses but is centralized to service for the Lord in this chapter.
but be transformed: Verb μεταμορφοῦσθε (metamorphousthe) meaning "change form", "transfigure". We get metamorphosis from this word. The verb is rendered in the present tense but again in the passive voice which means that the action is exerted by an outside agency not by ourselves.
by the renewing of your mind- Verb ἀνακαινώσει (anakainōsei) meaning "make fresh", "a new development". Again the verb is rendered in the passive voice. Titus 3:5 defines the agency of the "renewal" which is the Holy Spirit.
5 He saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit,
The Greek word ἀνακαινώσει does not appear in secular texts simply because it's use is new. It is ONLY used to denote the renewing by the Holy Spirit. No man can ἀνακαινώσει their own mind.
so that you may [
d]prove what the will of God is, that which is good and [
e]acceptable and perfect.= The verb δοκιμάζειν (dokimazein) means "to put to the test" "prove" "examine". This is the first verb in this verse that is rendered in the active voice. It is the first instance in which the agency is an inward agency. We can control the testing. We can control the testing and prove that God's will is good, acceptable, and perfect. We, however, can not dokimazein our own will but only that of God.
Basically the verse is saying that by our spirit of worship we can prove that the will of God is good and prefect. However, the renewal of our mind and the transformation are God's work. Do not be conformed to this world here is used to not to form a schema that is the same as the inferior schema of this sinful era.
So Titus tells us that the renewal of the mind is by the Holy Spirit, but not by ANYTHING that we can do or ever do. Paul is telling us to cooperate with the Holy Spirit post-justification in our sanctification. However, the process of sanctification takes a lifetime and it progressively removes the power that sin has on the believer. It does not, however, fully remove the presence of sin which can only happen a after physical death.
I fail to see how this verse supports your "celebrating birthdays are sinful because the practice is purported to have a pagan origin". None of us that celebrate a love one's birthday is conforming to the sinfulness of this age. To say otherwise is to judge others outside of the teachings of scripture and to introduce a "Pharesee's like" legalistic stance. When I celebrate my love one's birthday I am thanking the Lord for the blessings that He has bestowed upon me and my family. I am honoring the physical result of that blessing in His name. There is nothing sinful about that.