<PAGE>   1
                                                                    EXHIBIT 5.1

                     [ARMSTRONG TEASDALE LLP LETTERHEAD]


                               September 24, 1999



Enterbank Holdings, Inc.                            EBH Capital Trust I
150 N. Meramec                                      c/o Enterbank Holdings, Inc.
Clayton, MO 63105                                   150 N. Meramec
                                                    Clayton, MO 63105

         Re:    Registration Statement on Form S-3 of Enterbank Holdings, Inc.
                and EBH Capital Trust I

Ladies and Gentlemen:

         We have acted as counsel to Enterbank Holdings, Inc., a Delaware
corporation (the "Company"), and EBH Capital Trust I, a Delaware statutory
business trust (the "Trust"), in connection with the preparation of a
Registration Statement on Form S-3 (the "Registration Statement") to be filed
by the Company and the Trust with the Securities and Exchange Commission (the
"SEC") for the purpose of registering under the Securities Act of 1933, as
amended, preferred securities (the "Preferred Securities") of the Trust,
subordinated debentures (the "Subordinated Debentures") of the Company and the
guarantee of the Company with respect to the Preferred Securities (the
"Guarantee").

         In connection with this opinion, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of (i) the certificate
of trust (the "Certificate of Trust") filed by the Trust with the Secretary of
State of the State of Delaware on September 9, 1999; (ii) the Trust Agreement,
dated as of September 9, 1999, with respect to the Trust; (iii) the form of the
Amended and Restated Trust Agreement with respect to the Trust; (iv) the form
of the Preferred Securities of the Trust; (v) the form of Guarantee between the
Company and Wilmington Trust Company, as trustee; (vi) the form of Subordinated
Debentures; and (vii) the form of the indenture (the "Indenture"), between the
Company and Wilmington Trust Company, as trustee, in each case in the form
filed as an exhibit to the Registration Statement. We have also examined
originals or copies, certified, or otherwise identified to our satisfaction, of
such other documents, certificates, and records as we have deemed necessary or
appropriate as a basis for the opinions set forth herein.

         In our examination, we have assumed the legal capacity of all natural
persons, the genuineness of all signatures, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as copies and the authenticity of the originals of
such copies. In examining documents executed by parties other than the Company

<PAGE>   2

ARMSTRONG TEASDALE LLP                                         Attorneys at Law
-------------------------------------------------------------------------------


Enterbank Holdings, Inc.
EBH Capital Trust I
September 24, 1999
Page 2

or the Trust, we have assumed that such parties had the power, corporate or
otherwise, to enter into and perform all obligations thereunder and have also
assumed the due authorization by all requisite action, corporate or otherwise,
and execution and delivery by such parties of such documents and that, except
as set forth in paragraphs (1) and (2) below, such documents constitute valid
and binding obligations of such parties. In addition, we have assumed that the
Amended and Restated Trust Agreement of the Trust, the Preferred Securities of
the Trust, the Guarantee, the Subordinated Debentures and the Indenture, when
executed, will be executed in substantially the form reviewed by us. As to any
facts material to the opinions expressed herein which were not independently
established or verified, we have relied upon oral or written statements and
representations of officers, trustees, and other representatives of the
Company, the Trust, and others.

         We are members of the bar of the State of Missouri, and our opinions
expressed herein are limited to the laws of the State of Missouri and the
General Corporation Law of the State of Delaware.

         Based upon and subject to the foregoing and to other qualifications
and limitations set forth herein, we are of the opinion that:

         1. After the Indenture has been duly executed and delivered, the
Subordinated Debentures, when duly executed, delivered, authenticated and
issued in accordance with the Indenture and delivered and paid for as
contemplated by the Registration Statement, will be valid and binding
obligations of the Company, entitled to the benefits of the Indenture and
enforceable against the Company in accordance with their terms, except to the
extent that enforcement thereof may be limited by (i) bankruptcy, insolvency,
reorganization, moratorium or other similar laws now or hereafter in effect
relating to creditors' rights generally, and (ii) general principles of equity
regardless of whether enforceability is considered in a proceeding at law or in
equity.

         2. The Guarantee, when duly executed and delivered by the parties
thereto, will be a valid and binding agreement of the Company, enforceable
against the Company in accordance with its terms, except to the extent that
enforcement thereof may be limited by (i) bankruptcy, insolvency,
reorganization, moratorium, or other similar laws now or hereafter in effect
relating to creditors' rights generally, and (ii) general principles of equity
regardless of whether enforceability is considered in a proceeding at law or in
equity.

<PAGE>   3
ARMSTRONG TEASDALE LLP                                         Attorneys at Law
-------------------------------------------------------------------------------

Enterbank Holdings, Inc.
EBH Capital Trust I
September 24, 1999
Page 3


         We hereby consent to the use of our name under the caption "VALIDITY
OF SECURITIES" in the Prospectus forming a part of the Registration Statement
and to the inclusion of this legal opinion as Exhibit 5.1 to the Registration
Statement.

                                   Very truly yours,

                                   ARMSTRONG TEASDALE LLP