Your argument using Mark 16:16 is a very old and weak argument in my opinion. In the Greek, the words for "believeth" and "is baptized" are "aorist tense participles." The tense of these participles are actions that happen prior to the leading verb of the sentence, which is "shall be saved". In other words, both are necessary. Also notice that belief is placed before baptism in this verse. The order is also important. Belief always occurs before baptism in the New Testament.
The use of "believeth not" in the last part of the verse is enough to get the point across. Without belief, which is shown to be required before baptism, salvation can not take place. There is no need to list baptism in the last part of the verse as it would have been redundant.
Really?
If you are Catholic , I will ask if the infants have properly "believed "before they are baptized?
If you are COC if a man that came foreword in faith and repented and believed drops dead before his baptism ...does he burn in hell?
Here is the gospel that Paul preached
1Cr 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
1Cr 15:2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
1Cr 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
1Cr 15:4 And that he was buried, and that he rose again the third day according to the scriptures:
Not one word about Baptism in the gospel that saves .
Read the words of Paul
1Cr 1:13 Is Christ divided? was Paul crucified for you? or were ye baptized in the name of Paul?
1Cr 1:14 I thank God that I baptized none of you, but Crispus and Gaius;
1Cr 1:15 Lest any should say that I had baptized in mine own name.
1Cr 1:16 And I baptized also the household of Stephanas: besides, I know not whether I baptized any other.
1Cr 1:17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.
1Cr 1:18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.