Yes, what you believe matters! Your beliefs shape not only who you are internally, but how you behave toward others.
Do you practice love? That depends on what you believe love is and how it is manifested. Do you have a work-ethic? If so, what is it and why do you have it? Do you strive to be a good spouse and/or parent? Your convictions on good parenting and spouse-ing will inform your decisions in that arena.
“As a man thinks in his heart,” said deity, “so is he” (Prov. 23.7; cf. Lk. 6.45; Mk. 7.20-23).
Yet, many take the contrary view — that what you believe does not matter. Think of the irony in that. The belief that: it does not matter what you believe — matters greatly to those who believe it — and some of them even scold others who would insist otherwise !
In truth, on a fundamental level, everyone, without exception, is molded by what they believe, even if they believe in unbelief.
But not only do our personal convictions matter on a temporal basis, what we believe regarding religion also matters eternally. Indeed, what you believe about…
If you wish to “please” God and “come to” him, then you “must believe that he is, and that he is a rewarder of them that diligently seek him” (Heb. 11.6). This passage establishes two fundamental truths about God which we must accept as true:
(1) he exists;
(2) he holds mankind accountable, rewarding those who diligently seek him out.
First, the evidence of the creation itself (including our own existence) is more than sufficient for a reasonable person to accept the proposition: God exists (cf. Ps. 19.1; 14.1). Those who fail to believe this are “without excuse” (Rm. 1.20f).
Second, it is not enough to believe in his reality — we must also believe in his person (his identity  and revealed will). Jesus said that “eternal life” belongs to those who “know you, the only true God” (Jn. 17.3). Too, salvation belongs to those who “believe,” “love,” and “obey” his will (Heb. 5.9; Jn. 14.15; 2 Jn. 1.6; Lk. 11.28), as revealed in the Bible (Gal. 1.11-12; 2 Tim. 3.16-17; 1 Thess. 2.13).
Conversely, when the Lord returns, he will take “vengeance on those who do not know God (his identity), and on those who do not obey the gospel (his will) of our Lord Jesus Christ” (2 Thess. 1.8-9). Clearly, what you believe about God matters!
Christ warned: “if you do not believe that I am he, you will die in your sins" (Jn. 8.24). Believing in Christ is necessary for salvation (Acts 16.31).
Christ is the eternal God (1 Tim. 1.17; 3.16; Acts 9.20); the sinless, sin-atoning sacrifice (2 Cor. 5.21; 1 Pt. 1.18-19); he was raised from the dead (Lk. 24.46; Rm. 10.9); and possesses “all authority…in heaven and on earth” (Mt. 28.18). We must not only believe in who Jesus is, we must also believe in what Jesus taught (Mk. 8.38), both personally (Mt. 28.20) and through his inspired disciples (Jn. Jn. 14.25-26; 17.20f).
Hence, what you believe about Christ matters!
The Bible Matters
Receiving eternal “life” hinges upon “believing” the things that “are written” in the pages of the Bible (Jn. 20.30-31). It alone constitutes “the power of God unto salvation for everyone who believes” (Rm. 1.16). Its “message” is “preached to save those who believe” (1 Cor. 1.21; cf. Acts 15.7; Eph. 1.13; 2 Thess. 2.13-14; 1 Pt. 1.22-25).
The Bible teaches us about the plan of salvation (for details, see: “Your First Steps Toward Heaven”), the purpose and design of the church and our responsibilities to it (for more, see my series of articles on: “The Lord’s Church”), and about how we ought to conduct ourselves both before God and our fellow man (cf. 2 Pt. 1.5-11; Col. 3.1-17; etc.), as well as those practices we are obliged to avoid, if we wish to “inherit the kingdom of God” (Gal. 5.19-21, etc.).
By contrast, those who “twist…the Scriptures” through spiritual ignorance or instability do so “to their own destruction” (2 Pt. 3.16; cf. 14-18), for they are “accursed” (Gal. 1.6-9). In like manner, those who “reject” its message judge themselves “unworthy of everlasting life” (Acts 13.46); and those whose minds are “blinded” from believing the Biblical message “are perishing” in their ignorance (2 Cor. 4.2-4).
Thus, what you believe about the Bible matters!
Scores of men and women, through the ages, have suffered and died because they believed the Biblical message. Through great pangs and valiant courage, they passed that message on to others after them (cf. Heb. 11.32-40; Jms. 5.10).
Paul, speaking of his own suffering, said: “we also believe and therefore speak” — so that “death is working in us, but life in you” (i.e., he suffered, so that, through his preaching, you might believe his message and live) — 2 Cor. 4.8-18.
Even the angels are profoundly interested in the Biblical message — and it doesn’t even pertain to them (cf. 1 Pt. 1.10-12; Heb. 2.16)! Thus, if what you believe does not matter, then all their toiling and suffering to teach us that message must have been in vain.
To the contrary, what you believe matters, now and forevermore!
 More often than not, people express this position simply to dismiss some point of view to which they themselves do not subscribe. Rather than debunk the position polemically, they prefer to say, simply: “I don’t care! And neither should you (or anyone else for that matter)!” Hence, they reveal that they really do care about the subject; they simply prefer some point-of-view other than the one in question, but are not interested in either refuting it or defending some other viewpoint. Thus, instead of meaning: “what anyone believes does not matter,” the point becomes: “what you believe does not matter…but what I believe does!” In either case, this position is untenable, since, in the very moment you express the belief that belief does not matter, you have already shot yourself in the foot.
 As to his identity: he is the creator and sustainer of “all things” (Rev. 4.11; Col. 1.16-17; Heb. 1.3; 11.3); no other deity exists, except him (Isa. 43.10; 1 Cor. 10.4f; Ps. 86.10); he is the “God of Abraham, Isaac, and Jacob” (Mt. 22.32); God is all-knowing (1 Jn. 3.20) and infinitely wise (Rm. 11.33); he is all-powerful (Mt. 19.26); he is everywhere, filling both heaven and earth (Jer. 23.24); he is “everlasting” (Ps. 90.2), “perfect” (Ps. 18.30), dependent on no one (Acts 17.24-25), “holy” (Isa. 6.3), “just and true…[worthy of being] fear[ed]…[and] glorif[ied]…[and] worship[ed]” (Rev. 15.3-4), “faithful” to keeping his promises (1 Cor. 1.9), “rich in mercy” (Eph. 2.4) and “love” (1 Jn. 4.8); he is certainly “long-suffering” toward sinners (2 Pt. 3.9f), but will unleash his “wrath” against those who refuse to repent (cf. Rm. 1.18; Jude 15); as such, he is both “kind” and “severe” (Rm. 11.22). In short, he is the God of the Bible.