Mere membership in a Baptist church is not the only criterion. There are certain distinctives that, if denied, makes one other than a Baptist. Not merely the form of church government, and not merely believer's baptism or mode thereof, but the meaning contained in the rite.
Our Baptism signifies our identity in Christ on the Cross, and in His death. In other words, we testify of Christ with us, rather, we in Him.
It is through Him that we have received the penalty due our sin, and also through Him that it is impossible that the pains of death may hold us.
Romans 6:3-11
Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
For he that is dead is freed from sin.
Now if we be dead with Christ, we believe that we shall also live with him:
Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him.
For in that he died, he died unto sin once: but in that he liveth, he liveth unto God.
Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
Peter also pointed to this meaning in baptism, in allusion to the Ark. It is the 'like figure,' or eminent picture. Noah and his family entered the Ark. Judgement is poured out upon the world, and upon the Ark. But the Ark carried them through the judgment. Bore the judgement on it's own body in Noah's place. The judgment lifted the Ark and cast it down. And Noah and his family in it. But the judgment didn't come near any of them. It didn't touch any of them.
The judgment could not swallow the Ark, because of its design; because of its nature.
And so because it is impossible that the Son could be held by the pains of death, He prevailed over the judgment of God, and we in Him.
That's what we signify in Baptism. Christ is our substitute. That's the Baptist faith message.
Can you really say that Jesus did not receive our due in our place, and be considered a Baptist?